A lot of people don’t want to because once you’ve rented it out, the tenant can never be kicked out unless they break the law.
Do you want to sign up for someone living in your backyard for the next 20 years?
An ADU should be like renting a room in your house. Exempt from fair housing laws, and at worst a non-guaranteed lease renewal, if not a simple 30 day notice.
To me it feels like the sensible thing to do here is to either actually abolish the fair housing laws if tenants don't really need those protections, or have them apply to these new constructions as well since tenants do need them.
A stance in the middle makes no sense considering the intention of these laws.
a) renting out a room in your house (where fair housing laws already don't apply) and,
b) renting out a fully distinct dwelling unit, where fair house laws definitely apply and serve an important function.
ADUs are always going to be a small percentage of the overall housing market, so lessening housing regulations around ADUs specifically is definitely not the same as abolishing fair housing laws entirely.