It's not as different as you think. Base salary in minor league baseball, for instance, is ~US$1500 per month, and they only get paid during the season.
That obviously changes with service time -- and any service in the majors bumps it up disproportionately -- but guys who toil for 5 or 10 years in the minors without making it to the big club aren't getting rich.
That obviously changes with service time -- and any service in the majors bumps it up disproportionately -- but guys who toil for 5 or 10 years in the minors without making it to the big club aren't getting rich.