I was a bit confused by OP's post - um, surely the answer is that everyone else who isn't posting on MN frugality threads (i.e. the other 99.99% of the population) rented the more expensive flats? Those type of discussions are going to be a self-selecting response, people who didn't save money that way aren't going to post on a thread saying they did the opposite.
Your post I think is more accurate. I have a friend who always complains about how 'lucky' me and other people in our group are for owning our own houses, and I have to admit I think (but don't say), something along the lines of what OP is complaining about - because I DID stay either in fairly grotty house shares in less salubrious places or with my parents for longer than I'd would have liked to (despite being very grateful to have been able to), in my 20s, to save up money for a deposit, when she chose to live in a nice flat in a good area on her own. So in our similar circumstances I do think she could have made other choices if saving money was a priority.
However your post is a good reminder that it wouldn't be fair to apply the same to people younger than me. I'm in my mid 30s now, when I was in those 'grotty houseshares' 10 years ago a room was c. £300 a month in a nice city. Same sort of rooms now are £500-700. When I bought my house in 2016 the identical house next door was for rent for £600, which was already nearly double my mortgage, it's just gone back on for £995. Wages definitely haven't nearly doubled in the intervening time.
I'm on the fence about the avocado/netflix thing. Yes, a lot of it is hyperbole. A £8 netflix subscription is cheaper than a tv licence, good value for money if you watch a lot of it and isn't going to make any difference in your overall savings. A mobile phone is a complete essential in 2024. It's pretty much impossible to buy TVs that AREN'T smart, etc. BUT....my deposit was made up of lots of 'small' savings that added up. Bringing a 30p tin of soup for lunch in work rather than getting a £3 tesco meal deal every day. Making my own coffee. Doing my own nails. Rarely getting takeaways. Getting the megabus for a weekend in London rather than the train. Walking home for 20 minutes rather than getting an uber. etc. Getting a haircut twice a year and dyeing it myself. Pre-drinking at home rather than doing pub crawls. Keeping an out of contract phone with a £10 giffgaff rather than £70p/m on the newest iphone.
Whereas I know lots of people my age or younger who spend what I think of even now as an absolute fortune on luxuries (£15 on coffees and pret every single day, £200 cut and highlights every 3 months, £40 gel nails 3 weekly, £50 getting make up done professionally whenever they attend a wedding) takeaways and brunch twice a week, uber when it's raining rather than wait for the bus) then complain it is impossible to save for a deposit. But saving £200 a month for 3 years is £7200. In a LISA with the government bonus that's £9000. Plus interest - say at 4% is about £300. If you can buy with a partner that's a £18600 for a deposit for only 3 years of fairly easy savings, not living in complete miserly misery. Even if you save half that and take 6 years that's a 5% deposit on a £370,000 home or a 10% one on a £185,000 starter house/flat (which outside of the SE there are still many). So I don't think it's AS impossible as is claimed for absolutely everyone under the age of 35 to buy a house.