Just speculating here. Maybe they have big customers that communicated to them that they will choose them over the competition if they open more of their software stack. Maybe they are less afraid of being sued because they are the underdog and they believe that the authorities would not be kind to a behemoth suing them, or they believe that the behemoth would not sue them because of the bad press it would generate vs the settlement they would get.
I actually don't know. But I am sure they have good reasons, just like the corps that keep it closed also have good reasons. It is not "good" vs "evil".
I do remember reading that that is the case for AMD, maybe one of their open source devs said it in an interview?
They've also been doing it for a long time now and the risks didn't seem to materialize, which surely helps with all the fears that you mentioned.