Probably to prove it's unencumbered, liquid collateral. Which makes sense if you disregard all the other ways to determine that
(The risk of course being when I renew and it’s way higher)
Technically you can pay off some of the mortgage early, split a new loan account, borrow from that new account, and then use that to invest, and claim the interest of that against tax. So basically those with more money pay less tax, of course.