I worked all my life and never got a tax deduction for Social Security. So now that I am retired, why do I have to pay tax on my Social Security? The government owes it to me and shouldn't be taxing it!
I can't answer the "why" part, its just how the IRS tax code is written.
If the only income you were receiving was social security none of it would be taxable, its only when you have other income in addition to the SS income, that part of the SS income may be taxable.