I wasn't terribly mad about the money (if I owe $7,000 then that's what I owe, it was my fault), but what I was mad about was the fact that I had to do anything in 2021 at all. If the IRS knows enough to know what I owe them, then why exactly do I have to file, just to have them validate that the numbers I give them match the numbers that they have on file.
This means that if you don't report sales from your side so you can note the cost basis, the irs assumes a cost basis of 0 and calculates owed tax on the entire sale value - this applies to stuff like RSU grants as well, and trips up a great many people entering the tech sector ( especially young people who may not have much tax filing experience )
I legitimately don't know why brokerages don't all include the cost basis by default ( or why they aren't required to by law )
An example of where the brokerage wouldn't know:
You buy 5 shares of stock at $X and then a few months later buy 6 more shares at $Y. Later on you sell 3 shares for $Z. What is your cost basis? It could be $3X or it could be $3Y. Or maybe a blend of the two $(2X + Y)! The IRS thinks that you should be the one to decide, not the brokerage.
I'm sorry, but this response comes up every time, and every time the answer is "yes, that's why I said you should be able to just tell them it's correct." It doesn't have to be any more difficult than checking a box.
90% of people are just going to use the standard deduction (no, seriously: in 2019 it was 87.3%[1]), and for most people their filing status will only change a handful of times in their lifetime. It makes no sense to waste everyone else's time for what should be at most 3 or 4 yes-or-no questions.
[1] https://www.irs.gov/statistics/soi-tax-stats-tax-stats-at-a-...
edit: It's also possible you file before things are reported to the IRS about you.