You could say the same about litteraly every space faring faction or hero. Why didn't Odin stop Thanos for example.
Not really. A. I'm not sure Odin COULD stop Thanos. Hell, Asgard had enough trouble just fending off the Dark Elves. B. And they DID protect their client worlds. I'm sure if Thanos had directly attacked one of the nine realms or whatever then the Asgardians would have responded.
Granted, the MCU cosmic universe is a murky, murky place. Relationships between intergalactic peoples are ill-defined at best, mostly non-existent apparently so hard to say how closely tey follow what happens on other worlds. You could say the same thing about any Avenger who didn't show up to help a fellow hero out in a pinch, really. But CM is SUCH a "win button" character and seems to effortlessly move around the universe without any limitations that it seems odd that in 30 years of space faring she never seemed to encounter Thanos or any of his minions and just curb stomp them. Clearly the mothership recognized her prowess because they all turned their guns on her the second she hit atmo so maybe they did have some inkling of what she could/would do. The GOTG spent a whole movie or 2 trying to circumvent Thanos so I don't see why anyone would think they didn't do their part. Anyway, given how easily and quickly CM can deal with pretty much anything I hope they come up with a reasonable limiting factor for her other than "I don't care to do that right now".