All this is linked to the endemic problem in UK public services - shitness.....!
You can see exactly why it happens. From the social housing occupant's perspective they have got something for free (or at least for a lot less than it's worth) so of course they want to hang on to it and benefit their family from it for as long as possible.
My biggest criticism of the shitness in UK public services is reserved for the failures in administration that allow this to happen.
But again you can see why it does. Evicting people is high profile, it might cost votes, politicians don't want to run that risk.
Checking and enforcing housing rules takes time and energy. If you are a local authority housing official you don't get paid more for doing a bad job than a good job. So why bother.
People talk about the "blob" and this is just another example. There would be a huge wider benefit to society in ensuring social housing only went to those entitled to it. But on an individual case by case basis for a housing officer in a local authority it's just too much trouble...
We need a revolution in how we manage our public services to make them properly efficient.