Why would you install heat pump without properly insulating home first?
Take a structural brick house built with no cavity insulation, plaster walls, and a finished interior and try to get any amount of additional insulation into the walls. You end up having to destroy and repair the finished plaster walls, which is obviously prohibitive from a labor cost standpoint. Those projects can be (and usually are) done when remodeling/redecorating is already planned (so you don't double-pay for finish work), but are economically unrealistic to do just to save on HVAC.
>economically unrealistic to do just to save on HVAC
That depends on how much the HVAC costs... I saw some videos from last USA freeze where people had ice (!) inside of their homes - I doubt heating that is in any way economical.
Air sealing is tougher, but techniques like aerobarrier where an aerosolized polymer is sprayed throughout the home while it's under positive pressure has made air sealing fairly simple. Stuff like that can plug up to multiple inch gaps