I agree that it's a thing, aside from some examples I have in mind, like for example, the original authors of OpenSSL (originally SSLeay) didn't really ask for any credit as far as I know. I think you might be overestimating the number of people who do a lot of self-promotion simply because those people are going to be more visible. There are plenty of people who work on important things and don't really do a lot of marketing or self-promo.