It's actually kind of funny, because the answer is kinda simple. The reason is TV. In the 50s and early 60s, music was not "exposed"on the TV and thus you couldn't really tell who was doing what. In the late 60s, and thanks to the popularity of individual members of the Beatles, people wanted to know who was who. It spawned action figures, posters and a massive industry that was never seen before. Why wasn't it seen? Because it couldn't be seen.

That and, a lot of popular studio guys made really cool bands.