NAR was actually investigated by the federal government (FTC i think?), who concluded much the same, but lobbying buried the report and nothing came of it.
MLS's maintain power by strictly controlling access to information. they have the most up-to-date and the most detailed information about properties, buyers/sellers and sales transactions, among other things. this allows them to be the arbitors of deal flow, giving their members the advantage.
and this is the linchpin that props up the 6% commission rate. it's classic rent-seeking, since marketing costs have been dropping, as you mention. a truly competitive market would probably put the commission in the 1-2% range.
(i did my MBA thesis on disrupting the real estate industry)
edit: here's one, but i don't think it's the one i was thinking of: https://www.justice.gov/atr/competition-real-estate-brokerag...