Thats the thing, you double up on him and you leave other players free, so it's not that simple.
Great tactic if your play a sport where only Haalands goals count, otherwise it's really not a great tactic. In some ways it's worse doubling up on him. Even if you don't double up, he still has get the better of a defender. Surely the odds are better to go that route rather than leaving someome like Foden absolutely free.
The good thing is that this is clearly being highlighted now, so other managers have to choose between giving Haaland more room, which often means you'll get punished, or double up and let other top players waltz into your box unmarked.
That's why he's a huge asset even in games he doesn't score.