Why did scientist just assume?

[alpha_wolf]

How come scientist assummed that fundamental particles were point particles, that space was flat, that molecules were two-dimensional, and so on? If modern string theroy assumes string and branes have no thickness, why does it assume so? Doesn't it make more sence for particles to have volume, for molecules to be 3D like everything else around us, etc.?

[selfAdjoint]

Every now and then someone has tried to build a theory with particles that are little spheres. They quickly found that they couldn't make those theories work, and abandoned them. This was part of the cachet of string physics; it concerned elementary things that were extended, but didn't have the problems of solid particles.

[Stingray]

Every now and then someone has tried to build a theory with particles that are little spheres. They quickly found that they couldn't make those theories work, and abandoned them. This was part of the cachet of string physics; it concerned elementary things that were extended, but didn't have the problems of solid particles.

What sorts of problems did they run into?