You'd think that, but even simple algorithms exhibit complex emergent behavior when exposed to the real world.

How A Book About Flies Came To Be Priced $24 Million On Amazon: https://www.wired.com/2011/04/amazon-flies-24-million/

It's not that they're literally irrelevant as in have no effects; they're just irrelevant in the thinking of who you're trading with - no matter how complex the algorithm it's still buying or selling on behalf of a person or company.

> buying or selling on behalf of a person or company

Again, that's what you'd think and it's technically true that you can know who is running what system, but the systems are too complex to say they are really working on behalf of anything except the rules of min/maxing.

How do you explain bidding the price of a textbook up to millions of dollars is "working on behalf" of anyone?

Market makers just profit off of the bid ask spread, without “motivations” or “market knowledge”

Arbitraging the bid-ask spread is tied to both halves: motivation - desire to make a profit, market knowledge - knowing when there are arbitrage opportunities (asks smaller than bids).

Sure there are some closely managed algorithms but I think you underestimate how many algorithmic traders are ML shops that don't do anything other than "whatever makes the number biggest at the end of the day"