This is WHY: every exchange that holds users BTC also have theirs fBTC.
So exchange have 3 options:

• ignore fBTC - but users can be angry (at least)
• allow to withdraw fBTC by users - need some coding on echange and make zero profit
• start BTC/fBTC market - need more than some coding but also make profit
  So, exchanges that want profit (believing in fBTC futore or not) choose option 3.