My best guess is that it's a lot more costly than "keeping the lights on," probably in liability exposure.
You can eff around a lot in the tech space (in the US) on AI, privacy, data protection and preservation, reliability, etc., but when you get into the financial sector the rules change. The US's monetary system only works because it's audited out to hell and back to avoid people cheating. And the government is quite empowered to enforce scrutiny on that auditing. Even having a toe in the water in the US market probably opens up Google to massive investigative potential they'd prefer to just avoid.
"A strange game. The only winning move is not to play."
