Then either you only like Metallica and hate all other music ever made, or you've been largely ignorant to what's been released so far. I find it hard to believe that anyone could only like 3-4 songs out of 822 songs and who knows how many individual bands.
Of course people download rock band tracks. They recently passed 50,000,000 downloads. "Good" is a hard thing to measure with music. Everyone has different tastes. They try to keep variety in the game so that there's something for everyone.
You say "only 3-4 songs are good". I'd say that if they're releasing 3-4 songs a week that appeal to your specific tastes then they're doing a pretty damn good job!