Privacy? Ever fill out a 1040 form? Schedule C?
If you itemize deductions, your entire life is down on paper. Financial, medical, business. Remember, they can demand any additional details they want to justify a deduction.
Financial agencies--my banks, IRAs, etc.--are required by law to transmit my earnings directly to the government in case I don't remember to put them on my forms.
But hey, nobody holds a gun to my head, right? I could take the standard deduction and pay thousands more in taxes for the privilege.