Like people noted this is most likely a contractual issue between Valve and SE. It's the same for other MMORPGs as well, back in my ESO days I remember that the ZOS dev's commented that they really would like to offer that (ESO has the same problem) but that Valve doesn't allow that. Never, ever buy a game with an online account system (like MMORPGs) on Steam.

They really should add a HUGE disclaimer on the account creation page, but that will never happen ...