That’s true only for very simple situations.

Everything from upgrading your doors and windows or your air conditioner and getting a tax credit, to renting out a room and deducting that portion of your home expenses, to working a side handyman or cleaning gig, to spending some cold-wallet crypto to buy gold, to the cost basis of previously unreported gains, to getting married and deciding to file together - the IRS doesn’t know.

And some things are arbitrary, and you have to make the choice and then tell them what choice you made, and how you value it.