Depends. Do you think they were happy with their lifestyle?
I have a friend who is happily collecting his pension, has plenty of cash in the bank, but still works because he chooses to. He's a bit less frugal than your uncles but still lives well below his means.
Happiness for sure is a personal concept but I think they were in general on the "we came into these lives to suffer" boat. So, I think they were not "happy" for a healthy definition of happy, but they probably not even interested in being happy.
It would be a bit too long/detailed/personal answering that, but IMO a healthy definition of "happy" involves using the money you earn to make your life easier most of the time, during different phases and under most situations. For example, living in a house that is physically loosing pieces, that should be fixed to avoid problems, it will not generate happiness through consumption, but it will relieve you from stress of possible bad consequences. Not doing that when you do have the money indeed, it's not healthy.
Spending money on those things takes a lot of effort though, maybe they just had ADHD and didn't want to think about all the ways they could spend money?
Typically it is much easier to not spend money than to spend money. A trip to a foreign country is more effort then a local vacation, an expensive renovation is more effort than just living as is etc. A big home is more effort to clean, a big yard is more effort to manage, a big car takes more work to keep reasonable etc.
I have a friend who is happily collecting his pension, has plenty of cash in the bank, but still works because he chooses to. He's a bit less frugal than your uncles but still lives well below his means.