mortgage is a guaranteed 3%. That may not be much, but it is guaranteed and not zero (although inflation eats into that). Houses also generally appreciate in value.

MormonThug is right that most people don't have the discipline to put their extra income towards extra mortgage payments or investments. Most just increase their lifestyle. For a lot of people, a 15 year mortgage is a good way to guarantee that you're putting more towards increasing your net worth.

If I weren't investing anything and had the choice to put an extra $500 a month towards the mortgage or retirement investments, I'd probably pick retirement. More risk needs to be taken in that situation. If I were already putting at least 20% of gross income into retirement, I'd definitely throw it at the mortgage.

I don't know many people who have paid off their house and regretted it afterward. There is more freedom in having no house payment.