I'd disagree, Martial is having a tough season yes, but he scored what 23 last season? Rashford is currently on his second season in a row with 20+ goals, and Greenwood had a terrible start to the season, no doubt affected by the death of his friend and the media pressure.
Also, goals aren't created from thin air - one of the reasons why we've struggled is because we rely too heavily on Bruno, and teams know this, so they mark him out of the game more. When Pogba was injured we saw how much we struggled because teams would sit deep against us, and double mark Bruno/Rashford. They didn't even bother with the right hand side because they know whether it's Greenwood or James, it is unlikely to turn into a serious goal scoring opportunity.
On top of that if you watch Cavani's runs when he plays, he's constantly on the shoulder of the last man but we don't have the players to find him if Bruno is being marked out of the game. The same issue would occur with Haaland. Likewise with Martial, he's at his best with his back to goal, playing 1-2 passes with Rashford/Bruno to get in behind defences, again with Bruno marked out the game, and Rashford being half fit for most of the season it's no wonder he's struggled.
Haaland's best run of form has come from Sancho providing him with assists or being part of the build-up play, it's not like he's some demi-god who scores for fun out of nowhere. His goals usually come about as a result of great team play, either his teammates noticing his runs and giving him the ball, or his team creating space by playmaking as part of the build up process in order for him to get his shot off.