For people living in an expensive apartment, they might be barely making rent. Whereas they could obviously afford a mortgage for a similar monthly amount - but since they're living paycheck-to-paycheck they can't save the prodigious 20%. So they're stuck.
Of course perhaps some people have a nest-egg from a windfall or inheritance but then couldn't handle the monthly payment.
The main barrier these days to putting less down is twofold - one, you need to pay PMI which is just compounding the monthly payment problem. Two, good fricking luck getting your offer accepted with <20% down in these times. Sellers read into offers more than just "is this financed y/n" and there's IMMENSE preference for buyers who can put 20% down, because it signals that a) if necessary they could pay PMI and drop down the up-front % in order to close on a contract, and b) they probably have more cash than just that 20% and are only stopping at 20% because it is customary and they could weather unforeseen issues more easily.
To put it in anecdote form: I sold my house in 2020. We had 8 offers presented to us after 1 day on the market. All were for 20% down because my agent didn't even hear offers from people with less down, since we had so many at 20%. In mid-2023, my brother bought a house in a similar sort of area (metro area of a major-for-the-midwest city). 2 days on the market, he was one of 3 offers, all for 20% down and about 50k over asking (the house was priced somewhat low as a marketing strategy, but I'm guessing based on comps they still had to go in about 10k over a non-meme version of the list price). He wouldn't have even been in the running if he didn't have 20% down. And he also had to waive all contingencies which almost made his financing fall through/be late due to something stupid a random inspector said.
It's tough out there. Even if you can nominally "afford" a house, it can be hard to actually close on one in the open market without someone more financially established who is able to backstop you in various ways.
We would not have been able to afford a mortgage there because the only available housing stock requires too much work and $$$ to be worth it. We only bought this house because it was a fair price and all the HVAC was new. Nothing else in our market was acceptable at the time.
So basically, down payment is easy, monthly mortgage is ok, but any contracting work is a painful hemorrhage.
Assume you will spend 20 years to pay off the debt with 20% Down Payment. That means each month you will spend 80%/240 -> 0.3% of the house value each month. Including interest rate that's 0.4 - 0.5%.
So w/ down payment helps you can save up to 40-50 monthly payment.