It’s not that, it’s just that the accents in Britain and America diverged, with some of the most major changes being English becoming non-rhotic in a lot of England (this also carried over to America in some New England accents and AAVE)
There’s nothing inherently “posh” about Southern English accents, it’s just that Americans often perceive Brits as posh for historical reasons
In addition to all of that, since your comment is spot on:
When people claim some variety is more conservative than another variety, they tend to cherry pick a lot. It’s easy, for example, to look at rhoticity and claim “American English” is more conservative, or to look at the cot/caught merge and claim British English is more conservative. But neither claim is accurate or meaningful; and when you try to look at the big picture, you notice changes everywhere.
To complicate it further, neither “British English” nor “American English” refer to any actual variety. Those are only umbrella terms; they boil down to “English, arbitrarily restricted to people who live in the territory controlled by that specific government”. And the actual varieties that they speak might keep or change completely different features.
Our accents are also closer to old british before the poshies messed it into clownspeak.
It’s not that, it’s just that the accents in Britain and America diverged, with some of the most major changes being English becoming non-rhotic in a lot of England (this also carried over to America in some New England accents and AAVE)
There’s nothing inherently “posh” about Southern English accents, it’s just that Americans often perceive Brits as posh for historical reasons
In addition to all of that, since your comment is spot on:
When people claim some variety is more conservative than another variety, they tend to cherry pick a lot. It’s easy, for example, to look at rhoticity and claim “American English” is more conservative, or to look at the cot/caught merge and claim British English is more conservative. But neither claim is accurate or meaningful; and when you try to look at the big picture, you notice changes everywhere.
To complicate it further, neither “British English” nor “American English” refer to any actual variety. Those are only umbrella terms; they boil down to “English, arbitrarily restricted to people who live in the territory controlled by that specific government”. And the actual varieties that they speak might keep or change completely different features.