Digital Markets

Hello,

Why do some markets get digitized with direct business to consumer (eg books, cars and stocks) but others still need a broker in between (eg commodities, housing)?

It cannot be the financial size of the transaction, as banks do a lot without any humans involved. On the other hand, commodity markets are almost exclusively using brokers. Does it have to do with how standard a product it is perhaps? Although commodities should be quite standard…

I tried searching google scholar and scihub, but I couldn’t find anything that gave me any reasonable explanation.