Basically everywhere in the USA (and I believe most of the world) has restrictions on minimum house size, maximum building coverage on the lot, height limits, setbacks from property lines, room sizes, etc, etc, etc.
Mobile home parks are one way to have lower cost housing, but far from the only one. They are much less common than in the past, as they are being regulated and bought out of existence.
The restrictions are ones of practicality. There's a "minimum code" house you can build (that has the minimums required wherever possible) and it will be a certain price.
But things like utilities, etc will be a "per unit" cost not a "per square foot" cost, but selling the unit is pretty close to "per square foot" so the temptation is to inflate that as best you can to make the per unit costs as minimal as possible.
Mobile homes parks are a way to bypass certain slumlord laws; because the slumlord only rents the "land" and the mobile home is owned by the tenant, and lots of slumlord laws don't apply to your own home.
Utilities are a great point. Running water, sewer and electric to a house doesn’t get any cheaper past a minimum threshold that is suitable for a broad range of house sizes.
The only way to save on utilities is to “bundle” a bunch together as is done with a large apartment building (which often has one or two large commercial utility connections, with their respective costs and maintenance).
But for a given building “type” (single family home) the utilities are basically flat rate, with maybe electrical being the only one really increased, at not much cost.
Never heard of restrictions on minimum house size and amenities. Is this a widespread thing?
Would an expression of “smaller, lower amenity housing” be the standard mobile home neighborhood?