It's the same reason there are only a few phone numbers that can actually make my phone ring (my wife, the school my children attend, and pager duty (:P))
My device exists for my convenience.
Not sure if there's a better way for gmail, but the way I did it was to have a second gmail account, then create filters that send only important messages to that account. Then in the gmail app you can control notifications on a per-account level.
Could you describe briefly how to configure that state of affairs? What OS do you use?
Settings>Sound>Do Not Disturb>Priority only allows
You can limit notifications to starred contacts only for calls and also SMS, and then only star the contacts you want to allow.
My wife exclusively uses messenger for buying and selling stuff on Facebook. She doesn't need nor want notifications.
There's also the creepy factor. I assume if there is any kind of exploit or corner case that allows an app to abuse privilege on iOS, Facebook is scummy enough to always ship it.
Additionally, FB messenger does not allow you to disable "read receipts," it always shows the other party when you have opened a message. Buzzing my pocket and then telling the other person I read their message feels very intrusive to me.
If I don't have notifications enabled it's because I don't want to be distracted. I check my phone often enough, I don't need to immediately stop what I'm doing to read a message.
Equally. When I send a message to someone, I don't want to distract them either. Of course there's no sophistication (e.g. busy mode, notify for messages flagged urgent only) because facebook wants people to drop whatever they're doing and use facebook instead.