Almost no fantasy RPG let's a PC begin the game with an unlimited amount of money (eg as the first in line to a wealthy throne). There is no ingame rationale for this - no ingame rationale why an incredibly wealthy prince is never the protagonist. It's a metagame thing - to keep the power of PC's under control. And every fantasy RPG designed for ongoing campaign play tends to give advice of some form or other about the GM rationioning loot, even though such rationing is no part at all of the genre (eg Bilbo in The Hobbit).
You are correct in that the wealth factor is a game driven balancing mechanic. But what I disagree with is because of that, no flavor justification is required.
If a player of mine wanted to play a 1st level wealthy prince, one of my requirements would be: "Explain to me why you don't have access to huge amount of money right now".
The rules have placed some requirements on me to keep the game balanced, one of those is control on wealth. But that doesn't mean all my players are poor "just because". Maybe they were wealthy but everything has been stolen from them. Maybe they are secretly the heir to a throne but don't know it. Or....maybe they are just poor commoners. But spoken or not....there is a flavor justification for that mechanical rule.
And I think this is the place where people's opinions are divided on the subject. The question becomes, when does a rule require system assisted flavor vs just requiring the players to generate their own?
There are some cases where a player forced to justify their flavor actually creates immersion, and other cases where the justification actually detaches from it. I think it is the ongoing process to examine the areas in our DND culture, divide them into the two camps, and then provide flavor when flavor is needed.