Also: zoning tends to bottle up changes, so rather than gradually getting some 2-plexes, 4-plexes, and a few 6-plexes, you go straight from single family units

Also: zoning tends to bottle up changes, so rather than gradually getting some 2-plexes, 4-plexes, and a few 6-plexes, you go straight from single family units to larger apartments, which are more of an abrupt change. Where I lived in Italy, it was pretty normal to see everything from SFHs to 10-plexes, but big apartments with dozens of units were not very common at all.