No idea on the specifics, but the theory goes like this : clients have been burned many times over cheap solutions. Cheaper is usually more expensive on the long run. For this to break the circle you need reputation of reliability.
If you're in the US, restricting yourself to jobs where they are limiting to the US results lessens that problem (doesn't eliminate it - I suspect there's a ton of fronting accounts)
I've tried it on the buyer side and anything less than $50/hour can't speak proper English. It's definitively cheaper but not significantly; and if you add the struggle of going through all the profiles, then the price will match reality.
Upwork can work for very niche things as a extra channel. I've seen it work for people selling packages of "I'll fork bitcoin for you for 2k".
For normal contracts it's terrible.
I started on freelancer websites when I was 15 and it was nice to bag 20$ per hour plus extra if I happened to be faster.
But the more experienced you get the more you can charge and use your network instead of cheap jobs on Upwork.
How did you get started to snap up your first jobs on it?