One is that stock market grows faster on average. You make more money by investing.

Two is liquidity. It is much better to have money in brokerage account than in equity in house. The equity is harder to access when you need it, you have to sell the house or take out home equity loan. Brokerage account is fast to sell and transfer and can use for anything you want. A good example is can use the money for down payment on new house and wait to sell old house instead of having to sync the sales.

This assumes that you are the kind of person that can save and not spend it. If you can't then paying down mortgage is way to save money without having access.