I suspect its actually older properties that are more robustly constructed - the flats we had in central Edinburgh had 1m+ thick sandstone walls and we never heard anything from neighbours.

I'm currently in a 10 year old house based on old farm buildings and the walls here are even thicker (still sandstone).

Plaster vs. drywall probably has a lot to do with it. The plaster+lathe in my 103yo house (in the US) blocks sound exceptionally well.

The fairly large amounts of horsehair I used to find in old plaster probably helped with sound deadening as well!

On reflection, that's more likely. It's certainly been our experience of houses. The London apartment was a new conversion in an old building.

I've no experience of those converted or purpose built long ago.