Aye because we play football on the ground and not hoofed balls from goalkeeper to target man
. Making a pass to bring others in to play after shielding the ball is one thing ( something Martial does better than Lukaku) yet Lukaku is used more because he is the bigger guys, the taller guy who has gotten bigger to help him control the balls that we send over the top. To controle these balls and bring others to play and then make another run is much harder thing to do when balls are coming over the top rather than on the floor.
This is why Lukaku has struggled to make any sort of runs behind the defensive lines because he is focused with his back towards the goal trying to control the ball and bring others in to play from balls that need to be controlled in that way.
Firminho, Benzema, Messi are players who either run in to space created by other players or are players themselves who can bring inverted forwards in to play - as Benzema did with Ronaldo or how Messi did with david villa or with how Firminho brings Mane or Salah in to play.
Even if you disagree - tell me why in your eyes that our ONLY OPTION is to persist with Lukaku as a single target man (hence needing an additional target man) in comparison to playing with two strikers (from Rashford, Martial, Sanchez, Lukaku) and have them sharing the workload, the goal scoring burden inbetween them.
To make matters even worse - Playing with two strikers would cancel the need for us to go for a RW like Willian or Perisic as we can then have crossing abilities coming purely from wingbacks in comparison to whole 10 of the team crossing the ball to a single target man - whilst watching how it ends up being controlled like a coin flipping heads or tails.