First, it is frustrating. No argument there.
Having said that, I'm guilty of not following instructions myself, and I've been around since the game launched. For me, personally, it's due to the fact that I have difficulty dividing my attention between combat and the chat window.
It would be super helpful if there was an audio cue when messages came into chat from the team or league, but I'm not aware of any such cue. Then it would be easier to know when to glance down at chat. But for me, personally, it's really difficult to monitor both chat and combat at the same time.
YMMV.