Playing the markets can be pretty lucrative too.
Sure, but being able to get stuff yourself (for free) AND play buy low/sell high still beats only buying and selling and trusting in the supply to be there. There's a lot more bajillionaire end game characters than there are bajillionaire level twos.
Well, obviously, but correlation != causation. It takes a lot of time to reach bajillionaire status, most people are going to get some characters to endgame by then. You don't generally play the AH by actually just sitting at the AH for long periods, particularly not in games that allow addons.
You do need start-up capital. That can come from selling raw goods, from questing, or some combination of the two.
But, otherwise, most people who make money by playing the AH aren't wasting time gathering raw goods (at least in the games I play). Gathering raw materials has a much lower return on time spent, and playing the AH is a ton of profit for minimal effort (if you know what you're doing, at least). I can only comment on how it goes in EVE, WoW, and FFXIV, though.
EVE, in particular, is noteworthy, since Business, Industry, and Mining are all separate specializations with their own skills. Many people who play markets don't invest into Mining at all (because Mining is boring). I'd wager most invest into Industry (if only to get their own cargo ships), but I do know players who spend all their time PVPing and only invested into business to play markets, which gets them PLEX (for free access to the game), and all their ships/ammo/modules.
It's a preferred strategy, because Business has the best training time:profit ratio, and that means they can keep training their PVP skills instead.
Of course, plenty of players also roll Indy/Mining alts, instead, so there's that.