Besides, if you could earn miles with balance transfers, I'd get a card for American and Continental, and each month transfer the balance from one to another. If I had a $5,000 balance, I'd get 30,000 miles in each account without paying a dime. (Well, the annual fee for each).

Bottom line, the companies make more money from your purchases (which they make a guaranteed 2% - 8% from the merchant), PLUS your interest, than from a transferred balance, where they make $ on your interest only. Add to that there are a lot of people who transfer balances once the 5.9% or whatever rate expires.

A co-worker gets a new card every few months specifically so he can transfer his balance to a lower interest rate. When that rate expires, on to a new card.