Is that your personal experience?
I read almost only negative opinions about Upwork, and it's seems another broken solution looking for a problem to solve.
We've been growing, so all the good devs have been retained long term (12-24 months and counting). IOW, if you're good (decent tech chops + strong English communication), you can find long-term engagements via upwork in places that pay well.
Also, another downside: fraud. We ran into 2-3 folks who were operating outright scams, ranging from an agency posing as an individual, to identity / account theft.