Probably because the size of the commissions in fiat itself is larger. After all, banks charge a commission for everything: for replenishing an account, and for a transfer. Therefore, such a large percentage of the commission is obtained.
You should also consider their business model which consist of taking a few from the transactions.A lot of exchanges sets high fees to earn more and it's not bank's fault too.
I've stopped using changelly and its copycats long time ago because of the outrageous/ non-justified fees.