Many solar planes, like NASA's Helios (pictured above), NASA's Pathfinder, and to a lesser extent Facebook's Aquila employ the use of many small motor propeller sets instead of one or two engines like most planes. Why is this?
Smaller motors tend to have slightly higher power density, but aren't the relative losses higher on the smaller airfoils of these propellers?
Because they are very lightweight and fragile. Therefore, thrust and propulsion mass must be distributed over span - a single, large propeller and motor would put too much force into the structure locally. Also, it would require a higher landing gear to give the larger propeller the required clearance.
You are right about the higher losses. However, these airplanes were built to reach record altitudes, and wing loading must be as small as possible to make that possible. Also, boundary layer problems on propellers are less severe than on wings because the rotation will cause the slowed boundary layer to be moved outwards from centrifugal forces. There the blade moves faster which will very effectively suppress flow separation.
Conventional commercial designs try to maximize kg-kms per dollar fuel cost or kg payload per dollar investment. Or minimize operating & maintenance cost.
All these goals need an economy of scale that forces few large complex turbines.
On the other hand, most solar planes focus on raw, dollar-agnostic performance / endurance metrics like max height or max range etc.
This is an entirely different optimization problem.
Also, with higher payloads of conventional designs the structural weight of a sturdy airframe is already a sunk cost. With solar planes your baseline airframe can be very slender so every strengthening is adding weight.
Another fundamental aspect is that in solar the generation is anyways distributed & not centralized. Electrical distribution is relatively easier vs mechanical transmission.
In a conventional design it is difficult to get distributed generation (many tiny turbines & even if you did the fuel distribution system would offset any advantages) & no easy way to transmit mechanical power to multiple props unless you add a mechanical-to-electrical generation system.
PS. A lot of this is speculation on my part based on your very interesting question. I could be wrong.
Don't forget the electrical advantages of multiple motors. No need to conduct all the current to a single place with long lengths of heavy wire, no need to control a large current. Also, many small motors and propellers provide redundant depth/graceful degradation in case of a failure(s).
In addition to the thrust distribution, a multi motor design also allows you to turn off some motors if you run into a cloudy situation that provides too little power. One long bus tying all the panels together and each motor connected to it allows all the power to flow to a subset of motors. Once the sun comes back out, you turn all the motors back on and speed up.
This comes from living on a boat powered by solar panels and seeing just how powerful clouds are at turning 1KW panels in 200W panels. Being able to pare back the drain on the batteries is the only way to survive the "brown out".
