Because rooting for a shitty team can be one of the most soul crushing and frustrating things in the world, so fans use it as a badge of honor of sort, to make suffering a tad more palatable. Might not be the most rational thing in the world (though I do think it's totally worth it to stick it out through the bad times) but that's sports fandom for you, rationality isn't really a thing here.
Anyway, it's true that the Seahawks fanbase has a huge number of bandwaggoners, that's kinda to be expected though, it always happen when a team who was shitty for a long time gets hot. And for real, football is so big in the US that in engulf communities when the local team is doing well, so it brings casual/non-fans into the fold.
With that being said, I think the Pats had the single biggest bandwagon hopping phase I've ever seen in my life.
I know a lot of you younglings don't remember a time when the Pats weren't good (and as a side note, if you never rooted for bad team, it might be best not to grade other fans, but that's a different topic) but the pats were at one point one of the worst franchises in all of sports, and sports fans in Boston, which even I, who hate every Boston team have to admit are pretty legit fans, didn't give a flying fuck about it.
I mean there were fans of the team of course, but you routinely hear people say "pats? fuck that, that ain't Boston, it's a team for fuckers from New England".