For exchanges, I assume their business model requires free deposits and expensive withdrawals. That discourages people from taking their funds out of the website. For casinos, I kinda just assume the same! Why have someone withdraw 50,000 sat in LN if you can make him gamble more to reach the minimum on-chain withdrawal amount and pay a much higher withdrawal fee?
Fast and cheap withdrawals simply aren't in the interest of many businesses.

Or maybe I'm just wrong here, because cheap and fast deposits would totally be in their interest. It would be very nice if some casino owners can chip in on their reasoning