Unless you are a very important client who is paying money to the maintainers or unless the project is OSS but let's say, backed by/open sourced by a company like Google (as in, not some unknown person who kind of answers to no one) the chances of meaningful contributions landing are effectively zero.
If you are a client of a given library working for "Unknown company 8965688" and you are using a library and make a PR with a change you need or request it, in my experience, you'll simply be ignored.
Then comes the original post: we'll fork the library to an internal repository, make it our own, and stop depending on the whims of the maintainers who care absolutely nothing for the people who request features precisely _because they are actively using the library_ and need an improvement.
OSS has become a really, really bad place to be when it comes to actively depend on something that is managed by someone who simply doesn't care about you in the slightest.
Then, as we have people competent enough in the technology, forking becomes the natural thing to do.