Because there were more than seven people on the ship. This wasn't like TWOK where the ship was mostly crewed by cadets, or like TSFS where the ship was running on automation. There was a complete crew of qualified personnel on board, probably including dozens of people far more qualified to modify a torpedo than Leonard McCoy. In the time it took Spock and McCoy to get down to the torpedo bay, Spock could've just as easily ordered a weapons specialist to meet him there -- if there weren't several already there to start with. I mean, is it crazy to propose that the torpedo bay of a starship might actually be crewed with people qualified to work on torpedoes? And surely working with a weapons specialist who already knew what to do would've taken a lot less time than working with a medical doctor who needed to have every step explained to him as they went.