Search a US real estate topic. What do you get in top 50. Mostly Trulia and Zillow. Are these original sites? Maybe, but their data is taken from MLS. And many quality sites may never appear in a visible place. What can you do if you are in real estate business and looking for some organic traffic. Join an anti-T-Z group and promote it and get traffic from there? Did Google change significantly in last 3 years to maximize profit, maybe some thru hidden partners? Or maybe they are just improving their search engine. I don't want to blame anyone, and I don'tknow answers to these questions.
I think you are not the only who notice this phenomenon, but what we could do is just to wait and see how it work, where it goes.
Of course they are biased. They decide which sites offer the best experiences for their users for the search terms entered based on a number of unknown factors. The sites you mentioned are big and offer a lot of value for the users.