I think it's because it related to class not geography. And every other accent is. So take the thing I said earlier, which you agreed with "you can have posh Mancunian, or standard Mancunian"
I’m sure there are many variations, I’m not an expert on Mancunian accents.
Now take the Mancunian out. Now what. A "posh" what. What if there's no accent you can attribute to a place there. Now what if they're not posh either. Now what are they?
You are trying to have a rigid structure of subdivision across all accents where they all have to follow the same rules. Your logic above is not transferable across all accent groups.
Try looking at it the other way. Start from the sounds. A lot of people talk in a similar way. Ok, that’s a recognisable accent group. And then it’s a case of ‘what do they have in common, how do we describe this thing’. For some accents it’s clearly driven by locality. For others - such as RP/SSE there is a combination of geography (mainly out of the south east) and class/culture.
But literally all those people have to have in common is the way they talk. That’s their accent group. That’s your accent. Someone has put a label on it based on common factors in many people who have that accent. That’s all.
Yes, but every other English accent relates to a place. And if you don't have any of those recognisable regional accents, you are covered by this general blanket of RP, which actually doesn't relate to your class at all these days.
This is still, totally untrue. You couldn’t pitch up in Berkshire speaking with a totally different and heretofore undiscovered accent and have it be RP. RP is a specific, documented group of sounds used in speaking English. It just doesn’t categorise by geography.
Stop trying to make accents about geography. Just stop. Many are, but not all.
They’re a way of making sounds when we speak. That’s the accent. That’s it.
Everything else is an attempt to categorise.
(But also stop trying to pretend that RP/SSE is free of geography. It’s geography and/or class and/or other stuff.)