Yes, legal action is too much effort, cost, and time that most companies want to bother with, not to mention the above reasons. It's 2020, and RMT spam is still a problem unless they implement a player run chat filter, preferably allowing regex/wildcard. If it's too complex for people to wrap their heads around, all it needs is someone making a solid effort and the rest can copypasta line(s), and gold spamming'll be irrelevant. Atm, using/creating a third party filter is quite difficult to impossible due to the combo of gameguard/microsoft integration.
I don't know why most companies don't add a chat filter, I can only think that it gives negative connotations, maybe player abuse, and some probably want to deny that RMT is a real problem. The mmos that allow addons/have a built in filter, WoW/ESO/Runescape/BDO/etc...their gold spam is hardly as intrusive until it gets to say, mailing/invites/etc.
Sadly, supposedly JP PSO2 also doesn't have an RMT issue, gl trying to get Sega to do anything when their cares are Japan centric.