kxmode, I work in a development studio, and the simple answer as to why external beta testers aren't compensated is that they are willing to participate without compensation. When a big company suddenly finds itself unable to get any beta testers for its next MMORPG, that's when you'll see it break out incentives like free copies of the game.
Wide-open external beta testing is really only good for a few things:
1. Stress-testing for large-scale multiplayer games 2. Compatability issues 3. Free publicity and word-of-mouth
Unfortunately, #3 is a large part of why so many games now have external beta test periods. Internal, paid testers are the most reliable source of bugs and gameplay issues.