Remodeling is more difficult than new construction because you have to deal with everything that all the previous builders and remodelers have left behind, place all the wear and tear that the building has endured.  There is a saying that they don't build them like they used to, and while sometimes there is quality in older houses that you don't see anymore, its just as likely that there are things in the building which are far below the quality that we build to today.

Things you have to deal with:

Crooked things: walls out of plumb or with warps in them, warped floors, racked window and door fact anything that can be crooked usually is.  Roof rafters are sometimes undersized, floor joists are sometimes undersized, and rather than 16" on center spacing you find everything to be only in the ballpark.

Wiring is usually thin, there aren't enough outlets, the outlets aren't grounded, then panel is undersized.  Old houses were designed in an era when there were few electrical appliances.  In houses that have been upgraded you'll often find a mix of wiring, and wires that go to nowhere because they were replaced with others.

Plumbing is often rusted galvanized pipe, and if its copper, it usually was made with lead solder, and the pipes in both cases are run where ever was convenient even if that means hot water lines were run near the outside of insulated space.  Previous kitchen and bath remodels may have left capped pipes, or just unconnected pieces of pipe behind.

Insulation is often minimal, and if there was any retrofit in, sometimes it was done without considering the condensation issue.  Air sealing is non-existent.  If windows are double pane, they're usually not low-e.

Structurally, buildings may or not be good enough.  Even if they stand, all you need is a level to tell how much they sagged, compressed or otherwise moved.  In areas with earthquakes or hurricanes the construction standard has changed dramatically, and while new buildings are loaded with various metal components, old ones generally have none of them.

Got mildew, dampness or water in your basement?  You guessed it, old building generally didn't deal with water issues very well either.

Add to that a bit of rotting wood, maybe some old damage from termites or carpenter ants, the likelihood of lead paint just about everywhere, and maybe asbestos siding, or asbestos plaster, or asbestos duct liners or ... we just didn't know that asbestos and lead were toxic.

This isn't to say that new buildings are perfect; they're certainly not, and many of them won't last nearly as long as an old building already has.  However, most people grossly underestimate how difficult and how costly remodeling is.

In the future:

retrofit air sealing

retrofit windows & doors

retrofit insulation: blown in, adding rigid, thickening walls, adding rigid on the roof.

Whole house "gut job" remodel versus partial remodel.