there are several possible answers to your questions but here are a few...
there are more bands/artists/musicians because
- the population is growing so the absolute number of people in any given profession is growing as well (discounting the professions being replaced by new ones)
- easier to start a band, become an artist. some of the tools available today are very easy to operate so the barrier to entry is lower. of course, the barriers to excel are still high.
- exposure much easier to attain. there may have been a ton of garage bands before the internet but now they all are able to sell their music to the world without prohibitive distribution costs. so what might have already existed, is now not hidden away.
"sexy" industries will always have an overabundance of entrants.