It's a growth play for financial services to find trading volume they otherwise cannot create. Young retail has no cashflow to speak of, so Coinbase and Robinhood target them for the house rake of trading meme stocks and crypto. The rest is institutional, broker dealer, traditional retail brokerages, etc but there's only so much revenue to be had moving existing securities around amongst those who can obtain exposure to them, managing pensions and 401ks, etc. ~93% of equities are held by the top ~10% of US households, for example.

Lots of cost inefficiency to squeeze out, but everyone is incentivized to not cannibalize other participants or their own business. "Its a big club and you're not in it" and "If you're playing a poker game and you look around the table and and can't tell who the sucker is, it's you."

If I recall, the most optimal equities market operating model is a short auction window at the open and the close to process order books, but again, no appetite for efficiency over revenue, I'll have to find the paper.

(thoughts and opinions always my own)