Mortgages are fixed income. Stock market returns are variable and therefore riskier. One bad year can wipe out multiple years of gains. Meanwhile, the money you collect as interest has already been paid, and as you can see from the calculator, the interest is front loaded, meaning the majority of it is paid at the beginning of the loan. So even with the probability of a default wiping out the remainder that's owed, it's still a much safer investment.

Mortgages are "secured debt", meaning that they are backed by a collateral (in this case, the house). If the person defaults, the bank can seize the house. The risk is lower, and thus even when the interest rate is lower, the bank is willing to take it.