>And I'm sure these sites don't make it easy to figure out your total P&L because then you might stop.

That's just not true. Fanduel for example shows your lifetime win/loss statement ("career stats") prominently on your account page, including your Net Total in a large number right on top that is impossible to miss or ignore.

Most people lose, because you are paying a vig, so you have to win ~60% of your bets just to break even, but that doesn't mean everyone loses - they certainly don't. This is evidenced by the fact that people who routinely win from the sportsbooks can and do get banned.