Why would they? Isn't it impossible to beat blackjack in a long run with 6 decks and such?

Not impossible, but you do have to do some brazen, easily-detected stuff like watch the table for awhile and refuse to sit down unless the count goes positive.

What's that old line..."The markets can remain irrational longer than you can remain solvent."

Similarly, even though a player may lose asymptotically to the house, they can be quite annoying in the short run.