That's a nice idea but the thing is forum would have to verify that the account over there belongs to you too. If not scammers would have a way to fake the identity of an account on many of those exchanges just to use as an edge to scam people here and that won't be nice.

Another draw back is that there are so many exchanges out there and even if you pick the top ones some people still won't be able to make use of it since they may not have accounts on the listed one.

Aside from all these this forum supports privacy and Decentralisation and most users make use of CEX so allowing them add CEX to their profiles would look like the forum is partially promoting centralisation.