If you're trading more than $20 million a year in stocks, you are in the vast minority that everyone here acknowledges would need to adjust and file your taxes manually.

99.999% of the US population is not trading $20 million a year in stocks, and everyone else here is merely talking about the 95% of the population that has straightforward tax returns.

Cost basis reporting from brokers to the IRS has been around for several years.

Which is why most countries have a system that shows you what they think, and you can accept or not. It’s not like the majority of people are in such a situation, and likely over time that will become less common as its often a byproduct of legacy systems / mergers that lose the original transaction.