If such a site existed, how would you keep improvised tech-savvy teenagers from Bangladesh from joining it?
If you manged to keep teenagers from Bangladesh from such a site, why would anyone go there to overpay for fat, lazy, entitled westerner when they can get cheap, hungry Bangladeshians instead?
Plenty of people make good money freelancing despite lots of competition from cheap providers. They do it by being better at freelancing.
How exactly? Search this site (or google) for "freelancer".
You'll find discussions like:
https://news.ycombinator.com/item?id=4585435
https://news.ycombinator.com/item?id=5774303
https://news.ycombinator.com/item?id=9289500