Tools don't exist in a vacuum. When a tool is a closed platform driven by a business, that business is going to heavy-handidly steer how the tool is used.
It isn’t always socially appropriate to friend request people, which is why I rarely do it (because I have no idea what the other person’s expectations are). If I’ve been introduced to a total stranger via Messenger, I don’t necessarily want to (nor should I have to) be their “friend” just to have them see my messages.
Not everyone uses the tool the same way you do.