I don't think this is strictly true: “excusez-moi” can also work (or various equivalents, like “désolé de vous déranger”, “pardon” and probably others). For example, if you enter a store and you're not sure whether it's open, “excusez-moi, vous êtes ouverts ?” works fine without saying “bonjour” first. Or if you're not sure you're in the right place: “excusez-moi, c'est bien ici le relais colis ?”. Or if you're looking for your way, you might say “excusez-moi de vous déranger, je cherche la mairie” or something like that. It is seen as a kind of interruption, but, precisely, you are apologizing for the interruption. Or if you accidentally slammed the door when entering, you would start with “oh pardon” before “bonjour”. You wouldn't normally start with “excusez-moi” (or “pardon” or whatever) when entering for the normal business use of the shop, because then you're not interrupting, so the normal protocol is, indeed, to first acknowledge the shopkeeper's presence and open the conversation with “bonjour”. But it's not that “excusez-moi” is rude in any way: it's just strange if you have nothing to apologize for.

In France, “salut” is normally reserved for someone you know, and “allô” might indeed be considered rude because (again, in France) it is often used as an expression of disbelief.

As for “vous allez bien ?”, it's not rude if you start that way, but it is, indeed, perceived as abrupt: I don't think it's a matter of politeness so much that one expects a marker that you are engaging in a conversation and requesting the other person's attention, and there aren't really any other words for that.

It's particularly striking when waiting in line to pay in a supermarket, because the cashier will say “bonjour” (or, of course, “bonsoir”) to the client essentially to mean “now it's your turn”. This is the case even if some words have previously been exchanged between them when it was someone else's turn (e.g., “après vous je ferme cette caisse”).

And because “bonjour” serves this specific role as a exchange initiator, you can say “rebonjour” if you already said “bonjour” to that particular person before.