The made so much effort not to get caught. Secret signs, costumes, fake IDs, the whole role playing routine, and yet they made the biggest mistake you can POSSIBLY do: winning, winning, winning, winning and winning without end. Why were they so stupid?
shareOne interpretation of your question that's not been covered here is - why didn't they lose a few hands on purpose, if they suspected people were on to them etc.
In the book, in addition to the fact that, naturally, they lost overall on some nights/weekends anyway, one of the things they did to avoid detection was for the "big player" to deliberately make a few more bets after the deck had cooled - gives the casino a bit of money back, means they're not seen to be winning every time, and most crucially of all, after losing a few hands, the "big player" will be like "oh, it seems my luck's turned/my streak is over" etc, so it looks like he's not been playing any kind of strategy or counting.
Only daveyh actually addressed the OP's confusion in this thread
The BIG misconception is the lumping of all the players into a single "they."
The whole POINT of the MIT team's strategy was that the "winning players" were NOT grouped with the counting players.
Casinos spot counters by the way they bet. A single player, alone, who suddenly doubles his base bet for no clear reason immediately draws suspicion. When that player starts winning, the casino assumes he's a counter and bans him.
The brilliance of the MIT team's strategy was to have the "counter" and the "winner" be separate people.
The counter never draws initial attention because he's always betting the same amount.
The winner does draw attention but doesn't bet like a counter; he just bets a BIGGER base amount. And he leaves when he starts losing, which is perfectly normal.
The casinos' math told them that every winner eventually loses back more than he wins as long as he keeps playing. That's why some of the "winners" got pegged as whales who got free rooms and private jets from Boston to Vegas.
What's frustrating to me is this: the casinos would NEVER have caught on if the players hadn't ratted each other out.
No one in the casino business ever thought to notice who their "winners" were sitting with. Likely some racism was involved -- the counters were written off as "Asian small-betters" and ignored.
So to answer the OP's question:
(A) as daveyh points out, the DID "lose" on purpose from time to time, and
(B) they knew casinos were corporations that would mindlessly follow past practices.
Well, OP, you were sufficiently stupid to confuse “lose” with “loose,” so you can answer your question by looking in a mirror.
The fucking internet🤮