Not familiar with USA market but wondering: Are current houses just more fragile and fell apart within 50 years?
Obsolescence gets overlooked but it is important. Not only are houses often poorly constructed, but many are simply no longer desirable. For example in my area, anything pre-1960 is likely very small, possibly without a garage, and may only have one bathroom. The modern homeowner wants something better. Houses in the 1970s were frankly, weird. They don't fit modern tastes at all. Many houses built in the 1990s are falling down already because they were so poorly built. So my point is, you have to take into account not only the top end inventory number, but also that lack of desirability is a real thing that drags down actual available inventory.
You have no idea what you're talking about. Houses are built to standards significantly higher than they used to be (aka, with standards).
Contractors skimp where they can but, due to threats of litigation, those cut corners are cosmetics.
In the US, modern houses tend to be larger, have better insulation (from what I've experienced), but can be cheap in some aspects (e.g. interior doors tend to be hollow, whereas older houses typically had solid doors). Overall though, I'm not sure if new or older homes were better built. I know some really well built old homes, but some very poor built ones too, and this goes for modern homes as well. Most should last 50 years though.
Shitty wildcat builders throwing down entire subdivisions of deeply defective homes is a serious problem in the Phoenix area, I know that for a fact.
There are a lot of old abandoned homes. I would never live in a home built before 1978. Almost certainly that home is full of lead-based paint and asbestos.
Honestly unless the lead paint is the topmost layer, or they put so many coats of regular paint on that it’s falling off the wall, the actual risk of a toddler encountering and then consuming the paint chips is shockingly low.
We're closing in on 50 years past the 1979 lead-based paint cutoff - the vast majority of homes in my area, and I extrapolate that likely most areas, have been remediated. With asbestos - probably the worst issue in my area is that if you have asbestos shingles you want to remove they are stinking heavy and so cost a lot to dump. I live in a 1915 home that is a great example of houses in my area - has gone through multiple renovations in that time such that the guts are fundamentally new. Probably the biggest issue with these older homes is insulation, as unless you truly take them down to the studs, they are never going to be as airtight as modern homes.
One of my colleagues lived in such a multi-apartment block where there was a common toilet for the entire floor, so four households. In freezing weather, the toilet was bitterly cold.