I think it's because we don't have the quality players wo would make him sit up. Look at sane for example, he's got Bernardo, Sterling, Mahrez who could come to the wings and give him a run for his money hence he knows he had to keep working hard. If Lukaku was a quality top notch striker, believe me Rashford would keep working hard to rise above him but who are the attackers he can look up to? The lazy Martial who doesn't give a feck? Lukaku? Lingard? Sanchez?
That's why whenever a team has a quality 1st team players, the squad players would always want to work hard because without that, they would hardly see game times. You look at players like Pedro who doesn't have the talent of Sanchez, Messi on the wings while he was at Barca but he kept on working hard because he knew he had to keep up with the talents ahead of him and it paid well for him in the end but when you have shit players ahead of a talented player, the talented player wouldn't worry and would expect the coach to come running back to him regardless because the shit players ahead of him don't matter. That's the case of Martial and Rashford.
That's why I would love us to bring in Felix, Chiesa/Sancho and Ounas in the summer to give them a good competition. Moreover, the above players have fantastic work rates to go with their talents.