If you invest your money on anything that gives , say, 5% yearly returns on your money, instead of paying off your mortgage, which used to be at less than 2% (but is not getting beyone 4%), you're just better off.

Say you start with a 10,000 mortgage, and your payment per year is 500, not including interest. By the end of the year, you'll have a 9,500 mortgage left. If you just invested the 500 and only paid interest on the mortgage (which was common in Sweden until a few years ago), you'll still have a 10,000 mortgage, but you have 525 in your pocket. You could now go and pay off some mortgage, resulting in you having now only 9,475 left (i.e. you're better off)... but why would you do that? Just keep investing that money!