[AstralStorm]

It's not. Space shuttle was pretty well engineered to do the things it was meant to. (If anything, launcher and ops were underengineered hence the failures.)

The problem was that it was meant to do too many things at the same time. Some of them were: being able to return to US territory from anywhere. (Hence semi-aerodynamic.) Carrying people and a lot of cargo. (Hence big.) Deploying any kind of cargo rather than defined operations. (Hence Canadaarm.) Being reusable. (but not the launcher...) Etc.

The chief problem of it is that it was first in a class research vehicle. And you cannot refactor rockets. (At least not cheaply. Prove me wrong Starship.)