The expense involved in building prototypes is prohibitive unless all the tooling goes on to be used in production models and there is no way to tell that in advance. Computer aided design can help in testing many ideas. It's a lot cheaper to change ideas in a virtual computer simulation than in an actual setting. It just isn't practical to test an entire new design in real life. Sure, a sub-frame can be hand welded easily enough for a test but altering casting dies is probably out of the question.
