We just grow up with it because it’s how our parents and the parents of our friends speak.
If you want to change your accent you can, of course, get elocution lessons but most Brits do not. We just have a large variety of accents of which RP is one.
How so?
You don't have to follow them, but you do you should be ready to accept the consequences of your choice.
Mind you, I'm not saying that standards must be followed. I am just saying the same thing I tell my kids:
- the standards are there, wishing they didn't exist doesn't invalidate them
- the reason rules and standards came to existence might or might not be applicable to our current context, but some people will expect you to follow them regardless.
- If a rule or standard seems silly to you, make your best attempt at understanding why people would still follow it. (Chesterton's fence)
- You are free to not comply to some rules, but always be ready to accept the consequences of your decisions.
- What your friends are doing or not doing is not reason enough for you to change your behavior or choices.
But not observing them does. There are standards no one in the world follows anymore. They may still “be there”, but are only used for mocking purposes.
> If a rule or standard seems silly to you, make your best attempt at understanding why people would still follow it. (Chesterton's fence)
The corollary to that is that anyone who rebukes anyone else for not following a standard must be able to explain why it exists. “Because it’s rude” it’s not good enough, explain why it’s considered rude.
If people act like a standard doesn't exist, then the standard actually doesn't exist, because that's the only thing that defines a standard.
Standards are not absolutes.