Why are all these Republicans not seeking or resigning their office?

That party seems pretty close to implosion, doesn't feel like they have a solid direction or plan in place going forward and you have some traditionally red districts or states turning blue. The democrats on the other hand may have some weird ideas for what they want done but they seem to be, at least on the surface, unified in their objective and voice. I think some are getting out to preserve chances at future office because if their at the helm of a ship when it goes down they probably will never see office again.

The jig is up for both sides.

Because the double-edged sword of "transparency" has fundamentally changed the game. Think only a "few" have porn-stars, strippers and worse in their closets? The last decade of politics has devolved from a world-respected process to something more akin to a high-school prom campaign, with both sides looking for any and every "angle" to attack their opposition on. Most of these cats probably figured they were close to the "next slot" in the career track, only to realize running for office in the future would probably be best served by hiring an army of lawyers.