> What is shocking to me is how can so many boomers still have a mortgage?

If you have a low-interest mortgage then it doesn't make financial sense to pay it off any faster than you have to. I know many people that carry a mortgage they could easily pay down but choose not to purely on financial optimization grounds.

Yes as long as you actually invest the money you would otherwise be using to pay down the mortgage into something that has a higher return than the mortgage is costing you. This isn't difficult to do, but a lot of people don't do that, they just spend the money.