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Even at remote terminals as discussed in this answer it seems there's a preference for using a jet bridge to get passengers to and from the aircraft.
Can't the passengers just be bussed directly to the aircraft rather than the remote terminals?
Why the use of a jet bridge here if they can use stairs at other times?
The use of a jet bridge has several major advantages over loading passengers from the ramp via portable staircases or built-in "air stairs", among them:
Boarding of handicapped passengers
Not all passengers can use stairs: a jet bridge allows passengers with limited mobility, particularly those confined to wheelchairs, to board and disembark from the same level as the passenger deck.
Accommodating these passengers from the ramp would require some kind of lift mechanism to raise them to the passenger deck of the aircraft - this is typically accomplished elsewhere in the terminal when a jet bridge is used, e.g. via passenger elevators in the ticketing area, or in the type of design shown in your question, via an elevator in each "mini-terminal".
Passenger shelter
Boarding from the ramp may work well on a beautiful spring day, but in less favorable conditions (rain, snow, temperature extremes) most passengers would not find the experience to be enjoyable. The jet bridge provides an enclosed and relatively climate-controlled route from the terminal to the aircraft.
Passenger Safety
Airport ramps are inherently dangerous places: Aircraft are starting their engines, being pushed back for departure, etc. and ground support vehicles are moving at all times carrying luggage, cargo, catering supplies, etc. to and from the aircraft. In addition to the ongoing movement there is a great deal of noise from these operations.
Bringing passengers into this environment represents an inherent liability risk: Someone could be struck by a support vehicle, or injured by jet blast. A jet bridge is an enclosed area away from these dangers, providing an improvement in passenger safety.
Airport Security
A great deal of effort goes into ensuring that unauthorized persons do not have access to the airside facilities at an airport. Bringing passengers into this area represents a security risk, as even a ticketed passenger could be intent on causing disruption or damage once airside.
A passenger wandering off on the ramp would represent both a safety hazard (as the passenger could be injured) and a security hazard (as the passenger could perform any number of malicious acts before they are located).
Instead of built-in "air stairs" or portable staircases, Mobile lounge have been used for several airports to transfer passenger from terminal to planes directly.
Plane-Mate in Washington Dulles airport (Creditor)
However for several reasons most of the mobile lounge has been replaced.
Rise of air transport: Number of flights and capacity of airliners has benn rising quickly in these decades. It require more mobile lounges to serve the passengers. Furthermore the road network within airport have to be upgraded if there are too many mobile lounges. Transferring passengers by People mover or walkway is cheaper and faster.
Trend of airport operation: Since airport security has been continuously rising in these decades, tourists stay much longer in the airport than before. Hence, airport operator are likely to create more retail area so that they can generate more income. The concept of airport lounge does not fit this trend.
Though mobile lounge has been kicked by Jet bridge, it is still used for many airports for special customers, eg: wheelchair tourists to airliners in remote stand.
An additional complication not touched in the existing answers is the disparity in number of passengers that can be handled by an airliner and a bus.
A large bus might be able to accomodate somewhere on the order of 50-100 passengers comfortably while maintaining a reasonable turning radius and maneuverability as well as reasonable boarding and unboarding times, assuming luggage remains handled separately.
Large aircraft can handle far more passengers than that. For example, an Airbus A380 can take 853 passengers, and a Boeing 767 can accomodate up to 375 persons. Even the MD-80 can seat up to 172 passengers.
This means that you would need anywhere between four and seventeen bus roundtrips to get everybody onto the plane (two for a big bus and a MD-80), as opposed to simply moving everybody through a single jet bridge at the gate. Intuitively, this would seem to take far longer, in turn reducing aircraft turnaround and thus driving costs up. While the time to board passengers can still be used productively, I find it doubtful that the same could be said if the time to board passengers was at least several times longer.
For aircraft up to the number of passengers that can comfortably be accomodated by a single bus roundtrip, other issues (security, etc.) notwithstanding, boarding by bus remains a possibility without significantly impacting turnaround. But those types of aircraft tend to not be used for anything but short haul flights anyway.
The other answers address the question of why dropping passengers off on the apron by bus isn't common, but it does happen - mostly at smaller airports (for rural places with perhaps just one gate) and even at large busy hubs.
There are several reasons airlines sometimes board by having passengers walk to the plane and then either up a ramp or airstair:
Not enough gate space. This could be because a flight that should have left is under some delay and taking up a gate, an inbound flight is early, a diverted aircraft has arrived, or there are simply too many scheduled flights right now. For example, if a terminal was originally built to accommodate 10 certain size aircraft but the airline expands its fleet to use a larger aircraft, such that the terminal can now serve only 9 at a time, they might occasionally or routinely bus passengers to and from that 10th flight. This could also be a temporary situation lasting hours to months, caused by terminal construction for example.
Plane too small for a jetbridge. This is the case with some smaller turboprop or jet planes in use by airlines.
Other unique situations. For example, at LAX, customs in most terminals close after a certain time of day. An arrival from Mexico to T7 at night will involve parking at T7 and the entire passenger group and flight crew getting on large busses and taken to customs at the main international terminal.
