Running your own email server is definitely not "easier" if you are actually trying to maintain any durability and availability SLA's with your average homeowners computer/storage/network.

And sure, you can use git by itself and send the diffs to each other over email. However, then you have to deal with coordinating where the head is which is a pain across every node since it is constantly changing. Thus, most people don't do it that way and instead use a centralized service like github.