There are two prominent examples of antitrust/monopoly problems in the U.S. real estate market: MLS and dual agency. The MLS seems to be complicit in forcing home sellers into paying higher buyer agent commissions: https://therealdeal.com/national/2019/05/22/doj-demands-core...
I agree that being forced into 6% commission regardless of the effort it took to buy/sell your house is a problem, I don't think that extra few percent is what's pricing people out of buying a home.
The effective cost implications are more likely 10-12% at each sale. A seller will try to increase the price of the house to cover these commissions.
Your comment is focused on buying a home, but there can be dramatic consequences for home owners, the stakeholders forced into paying both agents.
When a seller has little equity and a poor housing market, exiting a primary mortgage can be financially impossible. A recent survey found a large fraction of the US can't handle a surprise $400 expense. So the follow-on effects of these commissions are not trivial for a large swath of the country.
I also agree that depending on location (looking at you SF metro) there are much bigger factors, e.g. constrained supply, inflating home prices.
With consolidation of real estate brokerage firms, many home buyers may have overpaid due to a single firm representing both home seller and buyer, known as dual agency: https://therealdeal.com/2019/05/01/houlihan-lawrence-fails-t...
There is a third issue of large private equity firms buying large volumes of houses that may, in the future, warrant an antitrust investigation.