The thing is, by sharing covers of popular music, there already is an audience that's interested in the stuff you upload before they even know you. They are listening to you because they know and like the original, rather than because you are a talented musician that knows about hashtags and algorithm. The fame was there before you and you just make good use of it. Of course I'm not saying that this is bad in any way.A buddy of mine that does music for shits and giggles on the side managed to get up to $1k a month in adrev royalties with some clever keywording. Then theres Lucas King who does mostly solo piano music and popular covers and has 650k subs.
You can get pretty far with the right keywords and an understanding of the Youtube algorithms.
I made a Godzilla OST cover some time ago which got far more views than anything of my original compositions, even though I think many of them are far more interesting and better (in my personal opinion). I'm not surprised about that fact at all.