I hire from these sites semi regularly, so looking a bit from the other side plus peeps I talk with who work regularly on these platforms.
I suspect it is hard to start. I tend to go for proven performers and even then it's full of low quality.
Some peeps I use do very well, are full on work (full time) with ever increasing rates.
I think the best strategy is to bid low, like crazy low. Yeah it sux but it's like starting your career again. You need to build reputation and take whatever job you can to get in the door. Make sure you get 5 stars. If you get a bad rating first few jobs I'd kill the account and start again as these are damaging without quality references to offset.
Approach with the restart mindset. Experience people who think they can join and get their real life career rate off the bat are likely dreaming. It seems a multi year commitment to build a network, and in the same way I filter 5-10 freelancers to get a decent one I'll keep working with, you likely need to do the same with clients.
And if I was trying to hack it, I'd ask existing clients if they mind using me on said platform to help build reputation faster.
And if your truly looking to work for top dollar, it's the wrong environment.