Most of it is that it's harder to come up with useful, safe, not overfitting APIs the deeper you go into internals. For example, registering extensions is rarely useful on its own as extensions are allowed to change literally anything else in the protocol, and without hooks for everything else you did not get much value.

But! Hearing about what people would like to do and then putting together an API if it makes sense is my job! So please do open an issue saying what you'd like to do but can't.

For example, I mean to extract Client Hello parsing and a capturing middleware net.Conn, as a lot of passive things can be implemented that way.