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