How come we have a set arbitrary number (50) of OU Pokemon?

I mean, is that really how the tiering system in Pokemon is supposed to work?

Currently we have a set number of Pokemon to determine how many are OU. I disagree with the system we have now that the top 50 used Pokemon in Standard are considered to be OU.

I thought that how tiering in Pokemon was built is that a Pokemon is bumped up to the appropriate tier if it "broke"/was considered too powerful for any tier below it. Therefore, the tier above UU should be any Pokemon that is simply too powerful for UU.

So under the current tiering system, that is BL. Which makes sense. However, the next part is what confuses me.

OU SHOULD be a tier composed of Pokemon that "break"/are considered too powerful for BL. However, under the current tiering system, we instead have an arbitrary number (50) determining how many Pokemon are considered OU. How did the number 50 (as in the top 50 most used Pokemon in an environment where only Ubers are banned) come about to decide how many Pokemon are OU? Why should it?

BL isn't a metagame. It is a banned list.

For someone who tries to speak so definitively about tiers you have no idea what you're talking about lol

Our tier list is not a set number. It is based on how often you would see a pokemon.

Also BL is not a tier, it is a ban list of UU.

I need to post this in a locked thread to make something clear:

OU IS NOT A SET NUMBER OF 50 POKEMON! ANYONE ELSE WHO SAYS THAT IT IS FROM NOW ON WILL BE INFRACTED WITH 'LURK MORE'.

Okay I'm done.