Ok, I will give you that London prices are silly, but it isn't the norm as far as the UK goes. And again I shared a flat, why does no one under 30 expect to do this any more?
Well, people live in flatshare a lot. I don't think you can ask why they don't do it when they do it ^^
One cultural thing I noticed in London and I assume in the UK at large is the lack of studios. There are plenty of 1 bedroom but there aren't studios (200-300 sqft).
If you go to Paris for comparison, you will see an incredible amount of studios.
It makes a world of difference when you get 25+, you make a living and you want to be independent (forget about dating and getting married otherwise). In the absence of studio, you're forced to pay up a lot to get a one bedroom (more than what low end jobs earn so they're stuck with flatmates).
Yes. I've seen what my younger sisters and exes earned working in London in jobs like waitress or saleswoman or law.
The salary simply doesn't cover rent and bills for a one bedroom. It doesn't matter how you put it, it just doesn't work.