Yes, they do offer transfer services. But do you trust their services to be flawless?
Like I mentioned, I've dealt with their brokerage-arms, and it was a disaster. Numbers being wrong between two large banks, the transfer taking _months_, and nobody knowing anything and everybody shrugging their shoulders and going "maybe call the other guys again?"
They have no incentive to make that seamless (in fact, they have every incentive to not make it seamless), and it isn't.
The same isn't true with your phone number. Once you've moved, it just works. You never have to tell anyone you're switching providers. The old provider is out of the picture and you no longer need to rely on their good will after your business relationship ends.