I kinda agree with you for the simple reason that the beta tests don't seem to serve much purpose. I have seen countless of major bugs being reported and still, they didn't fix them in the public release. So, one has to wonder really why they do beta tests...
I think that the betas serve the function of finding unknown bugs.
As a programmer, there are a few reasons why known (reported) bugs don't get fixed (during a beta cycle): Risk, Effort, and Priorities.
If a bug is high risk to fix and it's nearly time to ship, the fix should be postponed. Every code change has the potential to introduce one or more new defects. Unfortunately, I've learned this the hard way.
If a bug takes too much effort to fix (man-weeks or months), it should be postponed and lesser effort issues should be worked on instead (since you don't want to delay release by additional weeks or months).
Every defect is assigned a priority. Higher priority tasks include crashes and game-breaking bugs. You may not agree with FD's defect priorities (like for example, cockpit UI colors), but that's life: You don't get a vote.
Fixing bugs during a beta cycle is a balancing act. Fix what you can and defer what you need to. Trying to fix every known/reported defect will delay the release indefinitely.