I also live in Portland. Look into our local taxes for the answer. We have the highest local tax rate in the nation for families earning more than 200K a year, higher than Manhattan. It simply doesn't pencil to build an ADU and rent it out long term, so you’re incentivized to a short term rental like Airbnb, which does little to nothing to help the housing situation.

It’s the same for new apartment buildings. Next time you drive over the Fremont Bridge look at the skyline. 5 years ago there were 13 visible cranes mostly constructing high density apartment buildings. Today there are none. It’s the same reason, Portland has made it simply unprofitable to develop new high density housing.