There can be totally valid reasons for not getting rid of your mortgage even if you have the opportunity - if you have something better to do with the money. If you are paying 3.5% on your mortgage but you can invest somewhere with a return greater than 3.5% after tax, why would you pay down your mortgage?
1) It is difficult to take a house away from you if you are worried about "bail-in" type scenarios.
2) Paying off your mortgage is a guaranteed investment (there is no default risk) and so it should be compared with long term investing in very high quality government bonds when looking at rates of return, rather than what you can achieve more broadly.
3) It eliminates any future call risk, where your broader investments tank and it becomes difficult to meet your fixed mortgage obligation without liquidating investments at fire-sale prices.
4) It is perfectly hedged against future housing costs, which you will probably always need.
5) This is controversial, but if you believe, as I do, that a fractionally reserved monetary system is immoral, paying off your mortgage (and all the rest of your debt) is as hard a blow as you can land against it.