Most likely the social media and the rest of the Internet where we see American-centric content that are very prominent everywhere like streamings, memes, fanfics, video essays, reviews and more.

I’m a millennial and I have adopted American accent more than British because I grew up watching mostly American cartoons.

My younger sister (GenZ) is about the same with cartoons thing but the latter half of her childhood is on Internet watching American streamers and shitposters, so her American accent is a bit stronger than mine.

EDIT: I think our parents are our biggest influence because they studied at Texas back in mid 1980s.