You're just packing your bag wrong.
;-)
Strap something on underneath to level it out and voila, it stands on its own.
On a more serious note, I suspect the pack designers put all their efforts into the best carrying pack and/or the easiest to pack and organize. A flat bottom pack would likely not pack or carry as nicely as a round bottom pack, and unless packed just right, it probably wouldn't stand up anyway. The other alternative would be some added structure and thus added weight. Most of us would take the simpler pack in preference to the heavier, self standing one.
And, lastly, if you carry trekking poles (or find sticks), you can pretty easily create a tripod sort of support with enough structure to allow you to lean against your pack and use it as a back rest.
Wow, I do this all the time and can't find any pictures. It's kinda like the idea of
the chair shown in this picture, except you stick the poles up against your pack instead of into the chair pockets.