We don’t know why, but it’s just really not that socially acceptable to have your phone on loud all the time.
If you're in public and your phone isn't on silent, all eyes will turn on you if it starts ringing, as though you've just stood up and announced something earth-shattering.
Now imagine if you were just at home minding your own business whilst your boyfriend's WhatsApp, Messenger, iMessage, Snapchat and Instagram notifications are popping off day in, day out. It would get annoying really quickly.