They could compete in the SEC, and occasionally they have players like an Aaron Crow, who was a first round pick.
I don't know how much football talent is in that state though. I'm sure that there's some in the larger cities such as St. Louis and Kansas City, and maybe even a little bit in Springfield, Cape Girardeau and Jefferson City, but Missouri is no Florida or heck even a Georgia. In fact, I wouldn't be shocked if Mississippi puts out more football talent than Missouri.
The problem with Missouri is that they identify themselves as Midwestern. They care much more about beating Kansas and Illinois than they do, say Mississippi State. I think it would be kind of like when Arkansas joined the SEC and they still acted like they halfway belonged in the old Southwest Conference.