Togashi just wants to do it himself I’m pretty sure, he’s stubborn like thatAlways wondered why he couldn’t just do what Kishi and Mashima doing with Boruto and Fairy Tail: Hire an artist to do the drawing while they focus on the story. Kishi did it since drawing Naruto left him wrecked at his age and Mashima because he tackles so many other projects at once
Mashima was even able to hire someone who very easily emulates his art style too
He talked about it before with YYH where he was insanely burnt out and I think his injuries were starting to get the better of him, but he did like a large chunk of the sensui ending or something by himself with no assistants just for the satisfaction of it all