Extreme climbs like the Zoncolan or Angliru are mainly there to create interest and headlines. Races are rarely won or lost on these climbs, or at least no more often there than they are on other, less extreme crux moments in a given stage. At the Tour, every single rider is the best their team can offer for that role and every single rider is peaking for the season (if not for their career). Every day at the Tour is absolute brutality. This is not the case at the Giro or Vuelta, even though they are enormously tough races. The riders make the race, not the course.

Also, it's worth remembering that even the disregarded Mt Washington auto road in the hills of New Hampshire is harder than pretty much anything in a Grand Tour:
http://dsjansen0.tripod.com/Global_Climbs.html

And that's nothing compared to some of the really big stuff in Hawaii, Columbia, Ecuador or something like Onion Valley Road in the Sierra Nevadas. If you talk to pros who have done it, one of the toughest races in the world is the lightly regarded Tour of Quinghai Lake, which has an absolutely brutal course profile and takes place at altitude.

Also, the "official" second hardest climb in the world is in Taiwan, hardly a cycling mecca:
https://pjammcycling.com/