While you're right about the fact that many great developers aren't on the aforementioned sites, you are taking the case of exceptions.
The problem today is, for the majority of developers, who get day to day work done for products at various companies, to whom software development is just an employment, they are faced with a need to validate their skills.
This is a product for them. Most companies cannot afford the developers that you talk about and the developers probably aren't interested in working for them either.
And lastly, when a company scales, it is difficult to have hiring systems which can discern a good developer from a not so good one. Testing 100s of developers is tedious, even with SaaS products. We believe what we've built can save time, without having to compromise on skills.
But thanks for your feedback, appreciate the honesty. :)