> Most won't. My home is over 100 years old and I'm 100% positive it will outlast 90% of modern construction put up today 100 years from now. I've seen my house gutted in many places and it's pristine. I've seen many 20 year old homes that are full of mold and rot.
One of the main problems with modern houses is that they go to shit very quickly if they're ever unoccupied for a while. They need heating and cooling (especially the dehumidifying effect of AC) or they'll require extensive repairs and renovation within a year or two. Saw tons of these when looking at houses after the '08 bust—some of the bigger banks let houses sit for a couple years without even trying to sell them, and they inevitably had major visible mold problems, and probably a lot more going on inside the walls.
It's the sealing differences that do it. Modern houses are built to be super-tightly-sealed, but that means they have (at least seasonal) moisture problems without an active HVAC system. Older houses were built to breathe. Modern materials also aren't built or installed with major seasonal temp swings in mind, so you'll get things like badly-cracking drywall seams, buckling trim, buckling floors, fucked-up doorframes, and so on, due to thermal expansion and contraction. Even intricate, tight woodwork in very-old houses was crafted to survive that kind of thing (it had to be).
Not that older houses don't get into trouble without maintenance, but it takes longer and serious problems pretty much require a roof or wall breach. Modern houses will fall apart while the roof and exterior walls are totally fine.