I have been a Paradox beta tester (not to be confused with employee, so I don't speak for the company) since late 2003/early 2004. The cycle each and every time works like this:
100-150 beta testers are chosen. 30-40 bother to post in the first week, download the prototype/late-alpha build and fire it up. Crashes are found, people post general impressions. Then 2/3rds of THOSE people are never heard from again. Some of them (usually the more experienced betas) have legitimate reasons for being scarce: new jobs are obtained, illnesses happen, twin babies are born, all of which cut into beta time. These testers will show up a few times a week. Then there's a remaining 10 or so testers who HAMMER brutally on the game and the dev team. Features are revised, sometimes cut when it becomes clear that the design isn't as fun as actual gameplay, bugs get fixed.
There's right, there's wrong, and there's simple math. Devs who are busy coding and researching can't test everything. 10 full time beta testers and 15-20 casual beta testers can't test everything. Some bugs simply don't show up until 100,000 players hammer at the game and one of them finally presses just the right sequence of keys in the ledger screen. Sometimes something worked all through beta right up until the gold master build, and nobody knew to test it again because it had never been a problem before. It's easy to rip on Paradox and the betas, but you really should think about being one first so you at least know what is a fair comment to make.
+1