I think it very much depends on the area. In my ‘cool’ part of east London, housing stock is limited. Plenty of flats, with more being built, but houses are not being built, so for those young families who want to live in London rather than a suburb, the compromise is a smaller 3rd bedroom. All the Victorian and post Great War terraces and semis have a box room.
You can turn your nose up at it being too small, but plenty make it work. We used it for both kids when they were little and could realistically still have them in there had we not done a loft conversation to free up more space. Using this room gave us a spare room for guests and a desk for wfh.
In areas where the stock is similar to this, they have always been marketed as 3 beds. 3 doubles are much less likely without a loft conversion or in areas where the stock is larger (Edwardian or 30s semis) but they are often further out, or gentrified so long ago that they are £1-1.5m for a 3 bed.
I understand the frustration of not being able to afford your dream home, but I think some of your posts make you sound delusional. In desirable places, there will always be buyers. It might be that they have to take a 3 bed instead of a 4, or move a little further out, but in places like London, where there isn’t the land or the desire to build houses rather than pokey high rises, family homes will always sell.
Now, that’s not to say that I don’t think some sellers are mad - but it seems they often find buyers as mad. On a friend’s road, where the houses go for around £7-800k, one sold for over a million recently. It was beautifully done and no way would I spend £1m on that area, but someone did. All these delusional buyers only need 1 person who isn’t planning to sell anytime soon, and has a hefty deposit or full cash. And if the seller isn’t in a hurry, they may as well wait.
There’s a lot of sneering on this thread about houses not selling for months - but so what? If the seller doesn’t have to sell and is happy to wait for the 1 buyer, why not?