Give 5000 people that would buy the game a 50% discount and that's $125,000 less gross.
Consider that the discount comes before the retailer fees and it's an even harder hit for the manufacturer. The retailer still takes their cut of the full price, then leaves the publisher with the difference.
Especially considering at least 50% of the beta testers log less than 2 hours of play. If you're compensating someone you want to be certain they deserve that, which is why companies have beta testers on staff working set hours.
Fact remains that as long as volunteer beta testing as is popular and successful as it is (and it's very successful) it will always be around.