I assume part of this is because there is a lot of national forest not that far from Portland, and aside from a few exceptions here and there you can't buy a piece for yourself. So if you do want your own piece of rural wilderness you have to buy farther way. And Portland does have millions of people, many of whom have the financial resources to buy a little chunk of their own paradise.
Personally I'd pay a solid chunk of change for a campsite size chunk of land on the east fork of the Sandy River near Mt. Hood, right where Nottingham Campground is. I'd never leave.