The repo is backed up in your .git folder
Read again. "How often do you mirror your own repos on your local machine? Most of the time it's just your active projects.". Most of us have years worth of repos online, not a single one.
Which has no effect on your backups. Just causes a couple hour delay.
Some of us value our own time. A few hours of delay can be lost productivity.
In which case you can recover it through email
It's not that easy.
What kind of person would miss all their assets not being included by git?
People who properly setup git. Uploading large assets in git is stupid. That's why service like git LFS exist. Your default gitignore shouldn't upload assets.
Google, Github or Bitbucket don't suddenly run out of service. It would be a huge deal and you'd notice.
You never know. Those service are bought by larger companies all the time. Like Microsoft recently aquiring Github. You are on the whim of those companies. Ever heard about Sourceforge? Yeah...
What would be the time and place you need your backups but don't have internet?
A work place, Starbucks, commuting or even in the University. Internet might be too slow to download. Having an external drive with your backups is handy.
Doing useless stuff is waste of time
It's not a waste of time. Any person with a brain would automate the process. Losing hours or even years worth of work is a waste of time.
Like who