Part of it is just how emails work, part of it is how each clients work, part of it is people not knowing/caring.
You'll have:
- the guy that don't use the group email address as recipient, but personal email address
- the guy that change the subject which starts a new thread/discussion
- the guy that include all previous emails in their answer
- the guy with a signature that takes two screens to scroll
- the guy with an awful text font/color
- the guy that CC their whole address book, including the group email address, for personal stuff
- ...
I can go on. I went through this mess many times during the years, in various contexts; always the same result.