The biggest reason UAVs look like they do is because they don't need to carry a human, or the life support and avionics displays that human would need. The designs, therefore, are built around what the aircraft does need, mainly communications and viewing/recon equipment in addition to the standard stuff any combat aircraft needs (fuel, weapons, propulsion, lift/control surfaces). The use of a V-tail in practically all of them is for drag reduction; the aircraft doesn't have to be as maneuverable as a manned fighter, so a twin tail with separate all-moving elevators is superfluous, and the lower drag increases flight time for more range or loitering capability.
A key design consideration in any aircraft is weight balance. The 172, a fairly conventional aircraft design, is designed for a high degree of flexibility in its payload and distribution; the high wing keeps the center of gravity below the center of lift for stability, and by the same token the cockpit and cargo area are under the wing so changes in CG don't cause quite as drastic a change in handling.
In an unmanned aircraft, weight distribution is fairly static; any ordnance is right under the wings and thus the center of lift (fuel is also placed similarly), while the contents of the fuselage besides fuel load hardly change at all between flights. That allows the aircraft designer to allow form to follow function; the front of the aircraft has most of the avionics and communications, counterbalancing the propulsion system at the rear, with the fuel in the middle. The relative size and shape of these three elements is at the designer's discretion, and then the wings are simply placed at or slightly behind the balance point.
The RQ-1/MQ-1 Predator is probably the weirdest-looking, because in addition to having a V-tail for the weight and drag reasons, the tail is inverted:
The inverted V-tail has several advantages in a plane the Predator's size and with the complexity of the total flight system:
- A rudder surface placed beneath the CG, as on the Predator, will roll the plane toward the direction of yaw instead of away from it as a top-mounted rudder would, which allows for coordinated turns using only the rudder.
- This "rolling rudder" behavior is also pretty much all the RQ-1/MQ-1 needs in terms of roll control, so the rudder surfaces can be the only control surfaces on the plane, dramatically simplifying the control layout of the craft. Theoretically the lack of ailerons allows the wings to be removed or folded easily, but the MQ-1 has weapons pylons with firing circuits that complicate this. Larger drones with longer wings have higher MOI in the roll axis, more than the rudders could overcome by themselves, so these designs have ailerons (either wing-warping or hinged), which allows the V-tail to be upright for better ground clearance on landing.
- A crosswind on an inverted V-tail will cause the plane to pitch up and roll into the wind instead of pitching down and rolling away from it with an upright V-tail. Given the high comms latency between Nevada and Kandahar, this is an advantage, as the plane usually has plenty of room above it to fly while the pilot notices and reacts to the disturbance, but can only fly downward for so long before it runs out of air. Again, larger planes have higher MOIs and so disturbances are more minor.
- The inverted tail masks a little of the plane's engine noise in flight from specific forward angles. Not much, but every bit helps when you're trying to stay hidden.
- Lastly, the inverted tail protects the more expensive propeller if the pilot scrapes the back end on the ground. The larger Reaper has more ground clearance to allow for the weapons pylons, and so it can flare more on landing without involving the prop.
The RQ-2 Global Hawk actually doesn't look that farfetched; it bears some resemblance to the A-10 Warthog:
The two aircraft have vastly different mission profiles (the Global Hawk is intended as the U-2's replacement for high-altitude surveillance and reconnaissance while the A-10 is a venerable armor-killer close air support craft), but there's some common design elements, such as the high engine masked by the tail surfaces to reduce IR signature, and the big low wings producing a lot of lift (for a high ceiling in the GH's case, for payload and survivability in the A-10's).
What's not similar between the two is program costs; the original A-10As had a unit cost of just \$450k in the '70s, and with new wings and a glass cockpit upgrade the A-10Cs are still just \$11 million a frame. Unit costs of the Global Hawk, including R&D, make the program the most expensive small aircraft to date at \$222 million per unit, eclipsing the "cost is no object" F-22 (\$182m).
Other drones are much more cost-efficient; the Predator is just \$4 million each while the larger and more sophisticated Reaper is \$16 million. Losing one still hurts, and the USAF has lost plenty, mainly to operator error (the communications latency inherent in piloting one of these remotely from a ground station half a world away is considerable), but the total costs of these programs is still a pittance compared to any manned airframe in service today, with dramatically reduced maintenance to flight hour ratios and total costs per flight hour. The A-10 is again the cheapest manned combat aircraft in service, costing about \$18,000 per flight hour to operate (no word on what all that price includes; likely some combination of fuel, pilot pay, parts, maintenance and ordnance). The Predator costs just \$3,600 per flight hour, and the Reaper about \$4,800. Again, the Global Hawk program is fairly expensive to operate (about \$49,000 a flight hour) and that's a big reason the U-2 Dragon Ladies that Global Hawk was supposed to replace are still flying (the U-2 is only about \$30,000 a flight hour).