That's because there are so many low-bar employers.
Many of whom are also hanging out places like HN, complaining that there are no good developers while praising anything which drives down wages.
To know why there are few good developers, look no further than the market price. It doesn't justify top effort. I could sweat blood competing on price and throughput with people who have a cost of living that is a small fraction of mine, hoping against reason that someone will spontaneously give me a raise out of the goodness of their hearts. Or... not.
Most of the people who would eat your projects for breakfast don't want to be worked like pizza delivery drivers, delivering as cheap and fast as possible and hoping for tips. So they aren't on these sites. These sites are where careers go to die.