Their are many reasins why parents might want to stay in a house larger than other people think they need:-
They worked For and on it all their lives, paid tax on it, maintained , perhaps extended or improved it, repaired and cleaned it and got it just how they wanted it.
It is in the right place for seeing all of their family and friends.
They can put children and grandchildren up when needed, for example, holidays and Christmas.
Downsizing loses money in stamp duty and fees, it is often the case that downsizers end up with a two or three bedder for exactly the same money as the four bedder they sold. I know several couples who costed downsizing and found this to be the case so strayed put.
Downsizing means jettisoning beloved possessions too early.
Once you move from an expensive to a cheap area you cannot move back.
If you move to be near children they could move away again.
Adult children have rising incomes and can move upwards while grandparents are on small, fixed incomes and rely on their home as their only capital. Once it is gone it is gone.
The law of possession under the law is that, if a thing is yours, and you paid all the taxes due on it, it is yours to do with as you please.
No one I know from the generations before the present one ever looked at their parents’ house with envy or reproached them for being independent. They were grateful there might be some assets to inherit.
The present older generation is a prudent one who forewent much to prioritise housing. Some of them might not want to lose their independence to benefit children from a generation less prudent. I know a young couple who were saving for a deposit but blew it on a holiday and a wedding. How bitter to lose a house to hedonists who wasted your security in life on ephemera? I realise, of course, that many younger people are both grateful and prudent as is evidenced by many posters on this thread.
Many grandparents are contributing much to their offspring other than by giving up their house to them. It is estimated that free childcare and other subsidies from old to young run to billions of pounds’ worth per year.
I helped my daughter towards a deposit on a house by saving all her household contributions when she lived at home to work and by giving her much of my pension lump sum, saved for during 34 years teaching and she was delighted and grateful. She wants to move elsewhere in the country and for her DF and I to sell up to provide her with free childcare if she needs it. We said no.
I suggest to the OP that a three bed semi is not a small benefit in life and that many would envy her and think her attitude ‘entitled’.
Resentment is an emotion that poisons the life of the resenter and gratitude likewise enriches the thankful.