As far as I can tell, this distinction is mostly meaningless in practice.

Most people are going to want to sell their products on an international market, which essentially means designing and testing it to the strictest rules any country uses and just having to do a whole bunch of paperwork for the rest: getting a lab to do both FCC and CE is not significantly more expensive than either one on its own. And because FCC requires external testing, that means doing external testing.

Besides, although CE is technically a self-declaration that you follow the relevant rules, it still requires you to be able to demonstrate that you follow those rules - which means you have to test and report on a level comparable to an external lab, which means building a testing lab with a price tag comparable to a very nice home, and doing all the annoying paperwork like having your equipment regularly tested and calibrated. You are allowed to do it in-house, but is it worth it?