No it is not uncommon. There are a bunch of leading freelance websites that have invested heavily in their infrastructure, marketing, legal set up, financial set up and staffing. They have age, size and market share on their side and they all do a pretty good job in one way or another. The challange with a site like this is to get a good number of freelancers, to do that you need jobs posted. To get jobs posted, you need a good number of freelancers. It is like a catch 22 and until you meet the critical mass of both, you have nothing but an empty, non performing website. So it is a difficult market to enter.
Also, most freelancers avoid sites like these because of the 20% taken by the site. So as soon as possible, freelancers move off these sites when their reputation and client base grows.
Also, the prices these site quote are quite simply ridiculous in the extreme. One example I saw was for a website design and build, plus a food ordering and online delivery system, plus an admin for staff to run the food outlets from, plus an app to do the same but via geo location on android and apple, plus restaurant table booking, plus another app for waiters to take orders to tie in with the ordering system. Sounds like a great job eh, fabulous customer, tons of demands, tons of requirements, big project you might suspect. They were offering UP TO $500. I mean, honestly, I would not design or build a brochure site for that money.
Can you imagine the pile of crap they would get for $500 if you tried to do all the requested stuff.
Anyway, so yes, there are now big players left in a mature market, the numbers really show the difficulty of entry and competing to get any sort of market share.