I think the whole going up to 20th is one of those sacred cows in built to Dungeons & Dragons that fits the game's legendary status (struggling to think of the right turn of phrase). It's the same as alignment, beholders', chromatic/metallic dragons, vorpal swords...etc. When we think of D&D, at least when myself and people I know, think of a D&D campaign it is one of 1st through 20th. Taking that away would diminish the game. It wouldn't be the same game that we love.
In the likes of 1st and 2nd edition, we didn't push much into the teen levels because magic broke the game after a point. D&D has improved that. I think the game has improved in that regard as the editions have moved along and now D&D is in a good, more balanced, position to allow high level play.
As for why players these days aren't going the full way to 20th... it's going to differ by group and player, but there isn't enough high level material out there, even in 3PP or fan made PDFs on places like Drivethrough. I'm used to Paizo over the last 15 years doing full fledged adventure paths that take you from 1st to at least 16th if not higher in some cases. I may have to go back to some of the ones we never played and convert them to 2024.