"Actually explain?" Come on, read it. It says "casinos make money, but if you count cards, you make money." It's a worthless statement and it has nothing to do with the comment it replies to.
I downvoted dtby's comment because the reasonable way to deal with worthless comments is to downvote them, not post a followup saying "This comment is completely unhelpful."
In the absence of security countermeasures on the casino end, counting cards is not very difficult. You need to be able to do some mental arithmetic in a distraction-filled environment, and you need the self-discipline to follow the technique even when your guts are telling you otherwise, but lots of people have those character traits.
So if a casino does not take active measures to distinguish the blackjack players who are counting cards (who need to be kicked out) from the blackjack players who are just lucky (who need to be courted until their luck runs out), then lots of people, not just the generic “you”, will show up and gradually bleed the house dry.
I downvoted dtby's comment because the reasonable way to deal with worthless comments is to downvote them, not post a followup saying "This comment is completely unhelpful."