Because it's likely to cause discomfort and negative attention from the other passengers who will worry it could jeopardize their flight, and not using your phone for a little while isn't a big deal. It's not so much about following a rule or not, as just being socially responsible.
I agree, if it actually mattered then flight attendants would check for cell connections as rigorously as they do seatbelts. Even if you've never flown basic logic will tell you on a flight of 100+ people not everyone remembered to switch off their device. Trying to rat on your fellow passenger is the real offense here. Maybe I'm the jerk here but do we have to regulate our behavior to make nosy neighbors comfortable?
The argument goes that you have to regulate your behavior to assuage someone else's feelings, for the sake of getting along. This is a very good thing to do in many cases but the same argument still gets used even when other people's feelings are completely irrational. Why can't we sometimes just tell other people to get a handle on their feelings so as not to impose unnecessarily on my behavior?
At some point we have to put a cap on how much we value other people's feelings. There are neurotic personality types that by nature literally make up asinine things to worry about, and you'd have to run an infinite treadmill conforming to their nonsense. Empathy is an important value but its not an idol to be worshipped, and it should not be a rhetorical superweapon.
I mean if rules say you need to report your fellow Uighurs, you should follow them because other will?