OP. Life is complicated. Your are stereotyping, as a PP said: this is just like when awful people say that everyone on benefits are scroungers and lazy and wasting their money on booze/ fags. Simply not the case. Sure, there may be some but few and far between. I think your "£100k and should be rich but just mismanaging their money" people are equally rare, and just because you have met one family like that doesn't mean it's the norm of even common.
From the way you've described your views it seems you have little understanding of how the cost of living varies in different parts of the country. And for many earning those salaries, they have to be in those expensive areas to do so. Buying a cheaper house in the same area where they would be overcrowded would be expensive to do in itself. Moving away entirely means their job isn't doable anymore so they would earn less and negate the benefit of cheaper housing.
If you'd like to hear my own anecdata, here you go. Please do tell me at which point I was being reckless and mismanaging my money in an extravagant way:
I had been in a relationship for 10 years, living together for 5 of those. Then got married. Then three years after that we had a child and then another one less than two years later (as we had always planned: small age gap).
We owned a house together already at this point, bought just before we got married. Before the eldest turned two, he left us. He has no contact with the children. Unexpectedly, I found myself to be a lone parent.
After the second child was born I also became disabled. Despite this, I went back to work after a short maternity leave to provide for my children.
I earn the money you speak about in your post - over £100k. Mainly because I worked double the hours in my 20s that most other people did. But this income means I get no child benefit, no tax free childcare discount, no 30 hours funding etc. No help at all. Both of my children also have additional needs which means they cannot attend normal childcare settings for long hours so I have to hire private nannies even though one child is now at school. If I didn't do this, I would have to give up my job and be unable to house the children, the state would have to provide for us instead. Or, we continue as we are struggling to pay a large mortgage and huge childcare bills that eat 70% of my post-tax salary.
My health continually deteriorates under the stress and physical demands of it all, but if we were to move away somewhere cheaper I could not earn the income I do now. Especially as most employers are just crying out to employ lone parents with disabilities who need flexible working.
Do not judge people whose lives you know nothing about.