Cols are included in the ToF because 1) they are challenging
and 2) you can get ~200 cyclists, 40-50 vehicles & thousands of fans up and down them.
There are some stellar mountain roads in the Alps & Pyrenees that have never seen the grand tour peloton because they fail requirement #2. Wonderful riding - but no bragging rights back home.
Doh! smacks forehead There are - or were before COVID - some commercial tour outfits that offered fully supported col tours. No connection, not an endorsement, just remembered seeing these cats' ad.
https://bike-dreams.com/HC/EN/01_Intro.php