This is an oversimplification. The average household has several kinds of debt, and generally debt should be paid down in descending order of interest rate.
More significantly, investment income opportunities need to have their interest rate (or equivalent) assessed. For example, in the past 12 months, the DIA has risen 18%. Thus, if 12 months ago I had money to spare, it would have been better to put the money into DIA rather than make an extra principal payment on the mortgage, unless my mortgage is 18% or more.
With the exception of bonds and CDs, it's not possible to know the investment growth in advance, so that creates some risk of course.
Further, because of the amortization schedule, applying that extra principal payment only shaves off the last month of the amortization schedule, which is the smallest fraction of interest of all payments.
One would still be better off investing that money in something until that last month, then apply the payment to save the very small amount of interest.
This assumes the mortgage is like most (all?) mortgages out there that follow an amortization schedule -- which are unlike credit cards or student loans, where early payments have a big benefit.
Not necessarily – interest rates are exceptionally low in places. My own mortgage is around the 1% mark – it's more efficient to invest the money that would otherwise be used to pay it off.
(That's not to say that there are no other reasons to pay off the mortgage – owning a home outright is a great security to have.)
1% is incredibly low. Where do you live, when did you take out the loan, and what are the terms? According to this[1] chart, the lowest 30 year mortgage rate in the past 40 years was about 3.5%. I'm curious how you got a loan with such a low rate.
The UK. The mortgage market is different; terms are rarely 30 years, but rather homes are often refinanced every 2-5 years for a fixed mortgage rate, which reverts to a higher variable rate after that period.
Of course, this means that if interest rates suddenly spike then my mortgage payment will go up. But it does mean that in the short term it would not make sense to pay more off.
In Sweden the interest rates are around 1% (I don't know where the OP is from, just giving some context). I think the average is around 1.5% for all home owners and it's not unheard of for people with good jobs and not too big mortgages to have interest rates at the 0.5-0.75% mark.
This just isn't always the case, as others have said. Particularly with interest rates as low as they have been, it's often completely rational to pay off your mortgage as slow as you can.
Also, you might not want all your money tied up in your mortgage: Buying a second home, too much risk for your taste in real estate market, etc.
Besides investing in something with a better rate like others have said, you can also buy a boat and enjoy it while you're young then pay for it later when your salary is higher - even if it only grew to match inflation, that cancels out part of the mortgage interest.
More significantly, investment income opportunities need to have their interest rate (or equivalent) assessed. For example, in the past 12 months, the DIA has risen 18%. Thus, if 12 months ago I had money to spare, it would have been better to put the money into DIA rather than make an extra principal payment on the mortgage, unless my mortgage is 18% or more.
With the exception of bonds and CDs, it's not possible to know the investment growth in advance, so that creates some risk of course.