It could also be because said exchanges haven't designed an internal mechanism to adjust the fee of a user's transaction in the first place and just assigned a static constant for everybody regardless of network conditions.
Tracking the fee market and the fee rate is quite a simple thing to do and helps their user offset the costs of their transaction as well.
I'm almost certain that it is done for more profits and to a certain extent, helping the exchange to offset the costs of moving the funds within their on addresses as well.