So why does Tsunade still not create a leg for Might Guy?
If we want to create a leg for Might Guy, this means that his body needs to strong enough to handle the Hashirama Cells. Implementing the Hashirama Cells has his own risk. Currently Might Guy's body is not compatible with the Hashirama Cells because his body is not strong as before. If we force Might Guy to implement the Hashirama Cells, the consequences will be the same as Danzo.
Just like @TheTrue7thHokage said, Might Guy's body was totally destroyed and even almost died after the fight with Madara, unlike Sasuke and Naruto. (Only their hands were gone, not their entire bodies.)
Remember Danzo? His body also implemented the Hashirama Cells, but in the fight between Danzo and Sasuke, he needed to cut off his own arm in order to not let the cells devour him. At this state, Danzo is already very weakened.
As you can see with the red rectangle mark I have placed, this is the explanation for why Might Guy does not have his legs back. They are scared this case will happen with Might Guy.
Ok now let's talk about Naruto. Why can he can handle the Hashirama Cells?
At this state, Naruto is a very strong person: he has his Kurama's power, he has the Six Path Power, and he also knows how to use Sage mode. Obviously he is stronger than anyone in that world. So of course his body can sustain the cells.