They aren't slow, especially not at the World Tour level. They're just not quite as fast as the fastest climbers in the world. If the climbers are riding at or beyond threshold, then of course those with lower sustained W/kg are going to blow their doors off if they tried to keep pace.

I believe it was Michael Woods (EF) who mentioned during TdF commentary he did on FloBikes how literally every World Tour rider are among the best in the world at riding in all conditions and are the best in the world at one type (sprinting, flats, breakaways, rolling, etc). He mentioned being at one-day races wheezing up a steep climb only to turn and see Andre Greipel spinning calmly, breathing out his nose.

I'm told (since I don't pay attention to this sort of thing) that many Strava climbing segment KOMs in my area are still owned by Tyler Farrar, a sprinter and the last American to win a stage at the TdF, even though he stopped syncing his rides years ago.