"Major pro" in colloquial conversation is basically limited to the NFL, MLB, NBA, and the NHL in North America, which are the top tier of their respective sports and are by far the highest revenue generating leagues of their sports as well, not to mention that they are four of the five highest revenue generating leagues globally (along with the premier league).
MLS has some passionate followings and and has grown considerably in the past decade, but there's still a massive gulf between them and the minnow of the Big Four that is the NHL. And even if you think MLS should be in that same breath of conversation, others not considering that shouldn't be remotely surprising.
And to go to your snide comments about Illinois and Indiana in the NCAA, sure they are small by comparison to the mega programs, but they generate a helluva lot more revenue than midmajor programs, so nobody bats an eye at their inclusion as a power conference program (not to mention that Illinois is ranked 5th nationally in college basketball at this moment, so weird timing for a talent flex).
As for other Chicago-based teams... what? There are 20+ teams in each respective league that wishes they generated as much revenues as the Bears, Cubs, Bulls, and Hawks.
That being said, it's not a minor league, either. It's more comparable to the CFL than the minors or AHL. Not directly a feeder league, and it stands on it's own, but from a talent and money standpoint there is clearly a bigger level of competition available.