You trust your hard drive to never fail during crunch time, or Jane Doe in accounting to not lose the flash drive that gets shared around the office every quarter?

I'd take "generally highly available" vs "trusting my local copy" almost all the time. And as for using something like a Git'd Excel document: most people won't do that, and Github can go down, too.

Just because Google Drive isn't perfect, doesn't mean it's not better/easier/more user friendly than what has been the standard for years.