I wonder why there isn’t more roaming competition. Roaming users actually have a choice of carriers: my AT&T phone will never connect to a non-AT&T network in t

I wonder why there isn’t more roaming competition. Roaming users actually have a choice of carriers: my AT&T phone will never connect to a non-AT&T network in the US. In other countries there are usually 3 or 4 networks different networks I can connect to. (My phone is SIM locked, but I’m roaming with an AT&T SIM.) Why doesn’t one of those advertise to travelers and say “Use Carrier X, we’re 90% cheaper”. Is it because they can’t convince AT&T to pass on the savings?