That's exactly what the thread OP is complaining about - mom-and-pop homeowners choosing not to increase the housing supply by adding an ADU that they would rent out to someone, even though this is now legal to do, because they can't cope with the responsibilities of being a landlord.

Also, if a property did have an ADU in it, and someone was renting it out, a prospective buyer who didn't want the responsibility of being a landlord would find that a huge negative, which would decrease the value of the property to the existing owner, and provide additional incentive to avoid constructing the ADU at all.