It’s a much longer process than people realize—regional accents have progressively weakened over several generations. The accents you remember as a kid were likely less distinctive than the accents of a generation before.

The internet accelerated things, but so was the transition from blue collar manufacturing and agriculture to white collar services. Virtually everyone now sees themselves as middle class and 65% of millennials have completed at least some college, so a majority of people are aspiring to the sorts of fields where presenting as educated / professional is more important, and regional accents tend to be viewed as a sign of blue collar status.

What's happened more recently is that culture has globalized. But Americans haven't started sounding like they're from South Korea or Japan or the UK.

This is a dynamic, but the cultural dominance of the US has meant that more American vocab/slang/pronunciation has flowed to other countries than the reverse. Even the cultural powerhouses like South Korea, the UK and Japan have become more open to US culture through the process of appealing to and interacting with the giant US market. Those countries’ media markets are large but small compared to the US juggernaut. Blackpink’s Lisa learned English with an American accent to act on White Lotus, but I’m not aware of any comparably popular American artists learning Korean.