Because defenders have a much easier time when the main man they're up against is always in front of them wants the ball consistently played to feet. For them, it's like playing a videogame on easy mode.
Football teams don't routinely play a low block. There's a reason for that: it's a suicidal strategy against most sides. The exception is against teams like United, where our main centreforward is kind enough to stay in the defenders field of view at all times.
I'm not just talking about runs in behind (Rashford in particular does this reasonably well when we're on the counter). I'm talking about double movement, sneaking in behind defenders when they're not looking, running channels, starting offside and then coming back onside, etc. The dark arts of what it means to be a centreforward.
Chicharito wasn't anything close to a world class player, but the kinds of chances he scored from are currently getting ignored by this team. Creativity isn't a one a way street. It depends on the finisher as much as the passer. If De Bruyne or Zidane had nobody to pass to in dangerous positions, they too would struggle to create.
People criticise our other forwards for missing chances. But United's frontline is quite fluid and nobody has a fixed position. There's nothing stopping Martial from being the one who gets onto the end of those chances instead of Rashford, Greenwood, Bruno, etc. Ole isn't playing Martial like a sacrificial target man or a big lump battering ram or anything. The fact he has 0 (zero) shots on target this season tells you the story of what's going wrong.
My table on the previous page shows you that this isn't simply a one off. Yes, he's in bad form at the moment and I'm sure he'll get better. But his lack of penalty box instincts is a deeper problem, and one that's plagued him since day one.