They do this to avoid paying higher fees for their onchain transactions and thus increase their profits, but they can accept deposits even if it is $.
Most of the exchanges that I know have low minimum deposit except OKX which is 0.0005 BTC. With the current price of bitcoin which is $91500, that is more than $45. My friend fall for this legit scam while two or more people on this forum fall for this scam less than 3 or 4 months ago, including
BitMaxz. If you contact their customer care, they will not refund any coin.
In another thread, a user was able to recover his balance after sending less than 0.0005 BTC, so it is possible to recover it, but with a lot of headaches, so it is better to review the minimum limits.