There isn't a lot you can reasonably do to something that is already there. I insulated my attic better, but there wasn't enough space to go as high as I wanted (I guess I could in the middle, but not around the edges). The thin walls are still thin, and not much I can do about it for a reasonable price. Likewise the windows are really bad, but the cost of good windows is large. By the time I insulated this house to modern standards I'm nearly half way to tearing it down and building something new (a complete destroy is a lot cheaper than trying to take something off without destroying the rest) - and a new house would get a lot of other benefits (I want a larger kitchen but there is no place to put it)

Which is why a lot of poorly insulated houses still exist - people have mostly done what can be done for a reasonable price, but anything that will make a difference is also very expensive with very long paybacks.

Proper windows make a huge difference, too.

And then you need proper ventilation systems once you "fix" insulation