Beta playtests are performed exclusively out of house, with GMs and players unfamiliar with the game, as if they bought it cold at the store. Their purpose is to replicate the experience of a customer experiencing the product with no experience of the game at all, AND TO SEND FEEDBACK ON THAT EXPERIENCE BACK TO THE CREATIVE TEAM! (Emphasized for the 75 to 80% of beta testers who do not send a word back!) What they are testing is the way the rules are written - are they clear and concise? Do they confuse more than explain? Are they too long? Too short? Is play turgid? confusing? boring? fun? - etc. In short, Beta tests the expression of the rules.
Ideally, the package sent to the beta testers is fundamentally sound mechanically - that's the Alpha team's job - and problems with play should only occur due to poor choice of words. Unfortunately, the ideal is never completely reached, but it should be damned close.
As for the high proportion of Beta testers never sending back feedback, this is due to a lot of reasons. The best is that REAL LIFE just stepped in and flattened you. That happens, and there's nothing anyone can do. The worst reason is the tester didn't want to hurt the designer's feelings - Hey! That's why it was sent to you! I want that kind of hurt! I'd rather that hurt than the one where paying customers have problems!
A big problem is you can't just use the same good Beta Testers - the ones that give you good feedback - for every game. Beta testers get used to the way you write, and can guess what you mean. You don't want them to do that, as that shortcuts the whole reason for Beta testing! So a designer is always frantically searching for Beta testers, and praying that this one will be one of the few, the proud, the responders!