A simple, quick answer that I'm sure you heard is that global variables pollutes namespaces. The qualities of design choices are rarely apparent outside the real world.

A big problem with global is that it's often abused as a work around. Restricting access is an abstraction. The user isn't expected to alter this value, why should they be allowed to? What's more critical is the fact that the programmer might not realize what they wanted to be simply accessible is in fact static as well. So you now have a variable that's not only accessible, but state dependent. Now anyone using this variable has to be mindful of this.

Unless there is C code being called as well, in C++ you should rarely use global. It's much more manageable to have a game class object, where inside it, what used to be global could now just a private member that's global to that class.

You're team put in all that effort to remove global because it takes even more effort to get rid of all the trivial errors tied to the choice to begin with.

It all comes down to writing reusable code, objects that manage themselves. People shouldn't have to be cautious when reusing code. This doesnt only apply to other programmers but yourself 6 months down the line.