Nonsense. In most of the world you can tear down a bunch of houses, merge the parcels, and build an apartment building.
It might require permitting, but zoning is not an issue, because outside the US zoning tends to be based on activity nuisance levels.
In the US however zoning tends to be exclusive and a lot more specific, so you can (and cities do) zone large areas as single-family residential meaning you can not replace single family houses by MUAs without rezoning the area, a much more complicated, expensive, and lengthy proposition.
Not to mention US-style exclusive zoning also by definition precludes mixed-use developments.
