Bootstrap Sass as submodule

I just would like to suggest to use Bootstrap as a git submodule, to have its source fully in the project. Or is there a reason this is not done currently like that?

I'm considering using this material library, but what makes me shy away from it is the fact it's only "based on" Bootstrap as opposed to using it directly.

All modifications and enhancements for material can, in my opinion, be made on the Sass level by importing what is needed, swapping out the variables, etc.