Another component to it was that Haaland was arguably creating a dangerous situation by lifting his boot as high as he did trying to get to the ball before the keeper, particularly given the position of the keeper's "head" in relation to Haaland high boot. Haaland was attempting to make a first touch onto the ball there, and had he not lifted his boot as high as he did to try to get to the ball before the keeper did, the keeper may not have felt the need to reach out across the line before he otherwise would have needed to, which created the handball.
Had Haaland not put the boot so high, Henderson wouldn't have had to reach out when he did (beyond the line) to swat it away before Haaland got the boot to it. And if Henderson had not reached out over the line, had he pulled back lets say, where do you think the ball would have gone from Haaland's high boot? Surely not towards the goal. Based on the direction Haaland was attempting to kick that ball, had he gotten to it, it would have surely been kicked on towards the sideline, not towards the goal.