When the political class or the cultural zeitgeist tells you over and over that landlords are leeches and that "any attempt to profit off of housing is unethical"—people are going to take that to heart and have a hatred for even well-meaning small landlords. If you don't believe this is the attitude, go visit r/Seattle. The inflammatory language of politicians and cultural leaders sets the tone which plays out as legal battles and fights in properties across the city.
This obviously creates an adverse selection problem where small landlords illegally apply their own prejudices and biases in tenant selection. Honestly—could you expect them not to—when the repercussions of picking a bad tenant are so great? And when there are other demographic groups—like immigrants—who are absolutely, verifiably and consistently reliable as tenants. It used to be that it was the section 8 or low income type that were a huge problem but now there's an educated leftish fringe that landlords are also avoiding. Honestly with good reason, IMO.
Some homeowners just decide to not list extra rooms in their house outright. I remember hearing something like that Seattle has the highest number of unrented empty rooms in the country (though someone should fact check that). With the political climate the way it is here, it's obvious as to why this is the case.