The real answer is because the people involved believe the licenses they are using are even more free. IE they do not subscribe to the FSF's ideology.
It's their software, so they get to choose. It's like arguing NetBSD should use the GPL license.
As for why it generally doesn't make sense to use the GPL for this:
Even the FSF doesn't think GPLv2 is the right license for linux?
GPL'ing linux hasn't prevented any of the problems you mention, either in theory or in practice, so why do the same thing and hope for a different result?
(If you've never read it, i suggest you go read the various threads where linus, etc explain that in practice, the only approach that works is carrot, not stick)
It would be the same for Mozilla or anyone else who got popular enough with GPL software and had lots of vendors and OEMs who used their software.
GPL'ing the kernel or not does not change that some people don't follow the rules. You can have them sign it in blood if you want. They'll just work around you or ignore you, as long as there is money to be made. Also, once you are big enough, cutting them off will just attract regulators.
In practice, you will not get them to follow the rules by having a "war on gpl violators" any moreso than we have solved the US drug problem by having a "war on drugs".
If you (or others here) dream is of an ecosystem where everyone is forced, under pain of death, to follow the GPL, it's pretty unrealistic.
It's not what happens now, it will never happen.
Rather than pretend that's a realistic way to get people to release source, i'd rather not pretend. i'd rather see something different tried that may have some hope of success.