Any of the broadcasting companies with equipment located on Tiger or Cougar do care about listeners in Everett and Snohomish County. Cougar and Tiger are some of the best locations for broadcasting in this region, but unfortunately, there are dead spots to the north and south of the Seattle metropolitan area. In general, Cougar is closer to the metropolitan area and allows for better service in King County as a whole (as Kelly mentioned), with the only real exception being highway 18 behind the shadow of Tiger Mountain. The majority of listeners will be located in this county, therefore making technical changes to improve coverage in Everett or Olympia are not part of their consideration. While some of these stations may have reception problems this far away from Seattle, many people tune in anyway, and these listeners probably don't notice these coverage issues as much as those of us who are hyperaware might. Also, both Everett and Olympia have some of their own radio stations that are separate from the Seattle market, indicative that these areas are toward the fringe of the market where strong city-grade coverage isn't essential from a business standpoint. As for KUOW, they provide great coverage in the City of Seattle, but I agree, their signal is horrendous outside of the city. This isn't a television post, but it's the same story with KIRO, KING, KOMO, or any of the Seattle TV broadcasters using Queen Ann or even Capitol Hill.
I'm not an engineer, but that is my $0.02 for all that it's worth.