The ultimate problem is that of "trusted" individuals who go rogue.
And as for the "how could he not have been noticed sooner by the tower?", SEATAC is a large airport and multiple airplanes are always being moved on ramps, started up, stopped, shut down, etc. This would have been noticed earlier at a smaller, controlled field but so what? The guy could still have taken off before anybody could have prevented him from doing so. Stuff happens and nothing is 100%... which is why we have air defenses on alert.