1. They waste your time
Technically all games waste your time, that's what they've always done. But open world games waste the player's time like no other genre has done before. Loading screens to enter buildings, loading screens to enter missions, loading screens for fast travelling, pointless unskippable cutscenes, loading screens FOR cutscenes. And if there ISN'T fast travel, then you just have to walk all 3,000 meters over to your next mission, spending 20 minutes of your life doing so because we made the map so incredibly huge for no reason. Plus, you have to worry about things that might get in your way on the map while you're travelling there. Don't forget to pause the game to open the map and mark all the locations you need to go to. Don't forget to pause the game and go through your "checklist" quest log to choose a mission. Don't forget to pause the game and gather all the items and supplies for your weapon crafting. Oh look there's a thousand pointless collectibles on the map, try to find them all for the achievement! You could have just made your world a level selection screen at that point and saved the player all that time.
2. Unsatisfying
How long are open world games? 20 hours at minimum? 100 at the most? It's a huge time commitment, and for what? A story that you're going to have to refresh your memory on because you forgot most of it by the end of the game. Especially if you took a break from it and decided to go back later. The payoff can be good in the end, but what happens if it isn't?
The progression isn't satisfying either. Most open world games do the same barebones skill tree that you level up with skill points. Skill trees are the ILLUSION of satisfying progression. All they do is lock abilities you would normally have behind progression, and that's not fun. I have to progress your game in order to unlock the abilities that allow me to play it the way you intended? No thanks.
The movement and combat in open worlds are extremely unsatisfying as well. Traversal only consists of walking, running, and driving cars or riding horses if you're lucky. You're mostly holding the joystick forward for 10 minutes straight just to reach a point on the map, and most people abuse the fast travel system because getting around open worlds is just that boring. If you're going to make me move around to get to places in your game, then you have to make moving around fun. Traversal should be a game in and of itself!