Your question is much like asking why we can't afford to feed lobster to prisoners anymore.
True, but it seems the under 30 crowd would rather go to college/university instead these days. A degree costs about as much as a modest home these days. Almost nobody attended college in the 1950s. Hobbies always come with tradeoffs.
> And yes urbanisation has increased but not that dramatically.
15-20 points, at least in the USA. That is quite dramatic. If today's cities saw 15-20% of their population leave for rural areas, prices in those cities would drop like a rock.
> What hasn't really happened though is any significant new cities forming
Very true, and drastically increased farm profitability has driven farmland values through the room, which has made it nearly prohibitive for cities to expand like they used it do.
Fairly sure housing affordability is a bigger problem for those without tertiary degrees though!
> 15-20 points, at least in the USA
I'd think if there was an era where urbanisation was highest it would be in the 50-60 years prior to the 60s, and I'm not aware of house prices dramatically increasing in that time (happy to be shown otherwise though!). But in that earlier period many quite modest-sized towns grew massively to become the cities they are today (*), so arguably there weren't quite so many people all trying to crowd into the same few cities.
(*) though in Australia at least, something of the opposite has happened in the last ~75 years - many modest-size towns have lost their populations to larger cities.
US data suggests that college graduates have seen the largest decline in homeownership over the past 10 years.
That said, you are right that college filters out those with disabilities and other life challenges, and that means those without degrees are infinitely more likely to have something like a crippling mental disability which would prevent them from owning a home. Which, indeed, makes drawing any parallels between graduates and non-gradates rather useless.
> I'd think if there was an era where urbanisation was highest it would be in the 50-60 years prior to the 60s
In fact, there was a bit of a counter-urbanization movement during the period. Cities were considered to be for the poor. Anyone with means wanted to live on large estates out in the countryside. There was also that whole "White Flight" thing. All that goes back to the people not wanting the houses. It is true that things become much more affordable when they are considered undesirable.