Because when you take out a mortgage it gets deposited immediately in someone elses account. The mechanics are effectively the same as for non-intermediated (MBS) debt.
When a loan is securitzed (sold) by the bank, the DBE operations are:
[credit loan, debit deposit account of purchaser]
and the purchaser now owns loan. At this point strictly money has been removed from the banking system, but in practice the bank then recreates that money with a new loan (as above) because they are now within their regulatory lending requirements, and can issue a new loan. They can continue this indefinitely until the market for securitized loans dries up, which is effectively what happened in 2008.
IANA finance expert, but I suppose you could bound the MBS market with some kind of maximum-degrees-removed-from-real-assets metric, and regulate based off of that. But that, to me, would seem unwise since that metric would likely be fairly abstract and therefore dis-preferred as a basis of regulation.