There are often a lot of redundant parts of the legal & zoning code intended to limit development. You can legalize ADUs de jure, but you probably still have setbacks that de facto prohibit ADUs, you still have parking or curb or utility requirements that de facto prohibit ADUs, you still have HOAs that de facto prohibit ADUs, you still have "impact fees" and utility billing issues and all sorts of recently-added building codes that de facto prohibit ADUs. On top of that, the ADUs are usually extremely limited in character - "less than 1200sqft, less than 30% of the floor area of the house, no more than two residents", etc, etc. On top of that, you're paying for a financing category that the banks don't know how to deal with, without the subsidies that homeowners enjoy on mortgage payments, you're suddenly getting a property tax bill that's higher, and you're facing the possibility of squatting tenants under a regime that's designed to be inherently hostile to landlords.

You still have all sorts of laws against doing the thing you just supposedly legalized.