People are exerting pressure. Engineers understand what a PITA non-free drivers are, and what the vendors are missing out on by only providing an API, not instruction-level info, and I (and others) agitate within ARM every chance we get, so the people with the ability to do something about it are hearing the message regularly. But none of them have changed their minds yet. It'll take a while, same as it did on the desktop, and change is not inevitable.
Some people are scared about patents and loss of 'IP' control. Some believe their software is better than everyone else's so they don't want people copying it (failing to understand the difference between driver code and instruction set specs there). Several don't believe that outsiders could write drivers that were any good.