I choose to use frameworks in the same sense I choose to use crypto libraries. Smarter people have thought long and hard about the problems involved, and came up with the best ways to solve them.

Why have the agents redo all of that if it's not absolutely necessary? Which it probably isn't for ~98% of cases.

Also, the models are trained on code which predominantly uses frameworks, so it'll probably trend toward the average anyway and produce a variant of what already exists in frameworks.

In the cases where it might make sense, maybe the benefit then is the ability to take and use piecemeal parts of a framework or library and tailor it to your specific case, without importing the entire framework/library.