Adding a fixed fee to use the platform would lower engagement from occasional users or people that just want to try out the platform which would stunt growth.
While 20% is a large chunk to see just disappear, I think its a little extreme to call it oppressive. It’s industry standard (steam is 30% for example).
A reason why Upwork may undercut this by offering 10% could be that they are the underdog and need a reason to make you swap. I use Upwork too and the workflow is very messy there (from a buyers perspective). I’d much rather use Fiverr and pay the extra fees for the luxury of a nicer interface and high quality support.