You don’t want to do this now, but there’s a reasonable world we end up in where either inflation hovers at 1%, like most the last 5 years (so you earn 2% by paying down a mortgage, vs -1% when putting it into a savings account), or interest rates stabilize at 10%, or both!
Also some people just hate having debt, and so they want to pay down debts as quickly as possible. Even if it’s “irrational” their goal is to pay down their mortgage in the least amount of time. It is better for them to pay down principal than enqueue future payments.
Long term debt has a mental tax that does not show up in a bank statement. While it may be "smart" to pay the minimum amount on a subsidized student loan, there's nothing like just paying it off and being done with it, forever.
Answering both you and GP I belong to the “some people” group you described in your post. It’s not that I don’t understand the math, it’s just that I don’t have the mental capacity and time in my life to take advantage of this arbitrage.
To illustrate - I have an acquaintance who transfers money between credit cards to take advantage of points, and sign-up offers etc. Were I to engage in such activity myself - assuming I know anything about myself at all, it is that I will 100% forget some detail or date and end up getting charged all of the delayed interest making me worse off than if I didn’t engage in such a scheme at all.
So it’s like that. Rather than chasing some optimization in a half-ass manner, I’d rather just pay off the debt and pay less of my money to the bank.
There's no other thing to do, however, other than what you're already doing with your otherwise unused savings (hopefully index funds). Put your money in that instead of paying the mortgage early, and you'll end up with more money at the end of the mortgage. It's about the simplest optimization possible.
Also some people just hate having debt, and so they want to pay down debts as quickly as possible. Even if it’s “irrational” their goal is to pay down their mortgage in the least amount of time. It is better for them to pay down principal than enqueue future payments.