It's a bit more complicated. There is competition, but all platforms are slightly different and oftentimes it's hard for existing users to just go somewhere else when their whole portfolio is on Upwork and when their existing clients can't just switch to some other mode of interaction due to their contracts with the platform.

I think people have got used to pretty high cuts for platforms - e.g. app stores charge 30%. Patreon take 12% for premium plan. If Upwork were using Stripe then Stripe would be keeping up to 2.5% just for processing a card payment (leaving Upwork with only 2.5%).

Imagine getting a job via a recruiter just to get screwed the same way...