Because if the problem is bad enough, people will start looking for alternative places to buy things. It's like how people will avoid the bad neighborhoods in a city.
Oh right; if Amazon doesn't have what I want I'll get it on Walmart, the trustworthy online store!
Or BestBuy that doesn't have the marketplace problem.
But the once good, now bad neighborhoods can't buy the now good neighborhoods in monopolistic fashion; the analogy doesn't hold up with economics.