You mistakenly assume that simply knowing what is on the 1099-B form is sufficient to determine your tax on the gain. They don't know if you are married or single or head of household (filing status) in the current tax year. They don't know what some of your itemized deductions and other income not reported to them might be (which in turn, along with filing status, determines what marginal tax bracket you are in). They don't know if you are actually just a nominee for someone else's income. These are just a few examples. They don't know any of this stuff until you tell them by filing your complete return.