The population growth in Seattle each year exceeds the growth in housing supply required to house them, despite the level of new construction in Seattle. This has been the case for some time and will continue for the foreseeable future. There are not many cities that could absorb ~2% population growth year after year.

Even absent any non-resident speculators, the demand is still going to significantly exceed supply such that the prices increase given that housing is not growing at the rate of population. Foreign speculators may or may not make it worse but they aren't the primary cause of price increases; simple supply and demand is sufficient to explain it.