OK,
1. You apply force much more efficiently to the bars, no force is going in a downwards direction, but is all going horizontally towards the bars. Add in the effect of pushing off your opposite leg and 'power steering' as Keith calls it and you can apply some serious pressure to the bars.
2. It's much more aerodynamic.
3. Your weight is down low and towards the front of the bike, which is a much better place for keeping the front wheel on the ground.
It's actually not more tiresome to ride, once you get used to supporting yourself with your legs it's fine, plus unlike a naked where you get blasted with wind, the wind effect simply supports your upper body and takes even more weight off your wrists. As for looking ahead and to the sides, you can see straight ahead just fine, likewise to the sides.
Most of the downsides you list are merely practical ones which will be given zero consideration when the sole purpose of these bikes is to go as fast as possible.