How exactly is DJ Hero a Guitar Hero game? It plays different. It uses a different pripheral. You play to different music than in Guitar Hero. I don't see the connection outside of it having "Hero" in the title.
Regarding the rest, for them to manage this many releases tells me the dev costs are pretty low for each title. Different titles appeal to different people. Not everyone that wants Metallica is also going to want Van Halen. That's why they don't stick with the same types of bands for every band specific release. Every title released serves a purpose and a different customer base. Musical tastes are ridiculously varied, even within just rock music. This is why one game every couple of years isn't going to fly. What's great is that with a half dozen releases a year, it gives you choice as to which games you want to pick up. You don't have to just decide on the one game they come out with. You can find something that may better suit your personal taste in music.