MLB is swimming in revenues
OK, that last one's an exaggeration (nobody's got as much money as we borrow). But MLB is swimming in revenues and there's no end in sight. Someday, the poorest major-leaguer will earn a million dollars per season, and the typical major-leaguer will earn $5 million or $6 million or perhaps much more. Barring some sort of national economic collapse, it's simply a matter of time.
We're not there yet, though, unless you're an experienced starting pitcher, in which case there's really no limit to how much you can make, regardless of your actual skills or performance. It's a little like those infomercials arguing that you, too, can make a million buying and selling real estate, and the only real work involved is driving to the bank in your new Hummer. But this one's true.