I blame the mail clients. Most don't have a "reply to list" feature, so respondents end up "group reply"ing in order to make sure the mailing list features in the list of To: addresses.
It's lazy, and I hate it, but it gets the job done for them.
There isn't any way, for a given client, to ensure consistent behavior when replying to all lists, and for the lists there isn't any reasonable expectation that all users will have the same client behavior. Things are a mess all around now.
Mutt lets you define mailing lists, so if you press list reply (Ctrl-l by default) it will know whether to reply to the address the message was sent to.
It's lazy, and I hate it, but it gets the job done for them.