The modern day generalised British accent is the result of our upper-class ancestors making themselves sound more pronounced and proper which became a status symbol of sorts, and as a result everyone else began to adopt that. While of course anyone born after that change would naturally speak like their parents without having to force it, between putting on an American accent and speaking with my British accent, it feels way more natural to speak in an American accent. Each word flows into the next way more naturally and I don't have to pronounce almost every consonant.

It's been heavily theorised that the original British accent was close to if not the same as the modern American accent, something I wholeheartedly believe with how much easier it is to speak in the accent. That, and the fact that the British travellers that invaded America are now simply Americans with American accents. Would make sense if they retained the true British accent while us over here manufactured a posher sounding accent.

From personally researching this stuff I've also found that with old recordings, sometimes it's really hard to distinguish old American and British accents from each other (could just be from battered audio quality but I'm convinced that once we began recording human voices, we were at the tail end of the era where Brits and Americans sounded alike).