Why are such complicated time controls popular?
For instance, in the 2021 WCC
First point, such time controls are neither popular nor unpopular with the vast majority of chess players for the simple reason that we never experience them. "WCC" stands for "World Chess Championship". Only two players ever play that once every two years. In general the time controls you talk about are only experienced at the very top of the game by the small number of players who can actually make a full time living at the game.
Secondly, the time control you quote has a total playing time for the game of 6 hours 30 minutes plus 1 minute per move after move 60. That is a very long time to be playing one game of chess with no breaks. Yes, the players are allowed to go to the bathroom during that time and they are usually allowed to also drink while playing but eating at the board is not allowed.
The intermediate time controls provide natural break points during the game which allow the players to have a break, go to the bathroom, splash some water on their face to freshen up, grab a sandwich, etc. It is very clear why they are popular at the elite levels where players can be playing one game for 7 hours or more.
At the more mundane level that the vast majority of us play at the time control is 90+30, 90 minutes for the game plus a 30 second increment from move 1, so for most games 4 hours or less. We normal players do very occasionally go beyond 60 moves but it is rare compared to the top players. We can get by with no intermediate time controls.