Code Cruft in Zoning

Over time, any bit of code (zoning included) generates ad-hoc fixes to resolve problems never envisioned when the code was first devised, and most of those fixes satisfied an immediate need for a metric. Few of these metrics have any initial basis outside of simplicity and sufficiency, and their descendent metrics enjoy few advantages beyond custom and extrapolation.

Legally speaking, the purpose of all zoning code is to regulate health, safety and welfare. Functionally, zoning code has another use, beyond which: it is about pre-emptively managing nuisance by separating incompatible uses. Things which are noisy, or smelly, anything that disturbs a 'right to quiet enjoyment' are nuisances*. 

Even if we make the good-faith assumption that the purpose of all zoning code is to regulate health, safety and welfare, times changes and the problem that zoning fixes were designed to resolve is no longer relevant (ie, coal delivery, livery stables). 

Zoning is law, law is the basis of court cases and precedents, changing the law undermines the precedents, no city wants the cost & hassle of a lawsuit. So it's most often simply not to change zoning except on an as-needed basis. We often thing of the major cities like NYC or LA, but most city corporations are not far removed from that: In America, there are 19,000 incorporated cities, and over 100k 'populated places' of towns, villages, and townships. Many are not far removed from HOAs and are governed on a volunteer basis on a shoe-string budget. Hence, the quality of the average zoning code is not very good, and it is very likely out of date, most importantly regarding residential code.

Most of zoned land is zoned residential. Most of residential zoned land is zoned for single family detached housing, at a time when the nuclear family was the normal household, and land, gas and cars where cheap. None of these things are true any longer, but our zoning codes still reflect that.