Why do people on Social Security have to pay income tax on their Social Security Checks???

Up to fifty(50%) of Social Security benefits may be subject to income tax for individuals with a combined income between 25 thou and 34 thou, or for couples with a combined income between 32thou and 44thou and up to 85% of Social Security benefits may be subject to income tax for individuals with a combined income above 44thou!!! Less than a third of current Social Security beneficiaries have incomes that exceed the thresholds, requiring them to pay taxes on a portion of their Social Security benefits!! Why should they have to pay any tax when they already paid income tax on their money they earned when they were working!!! It looks like they are being taxed twice>Once when they worked and again when they Retire!!! That doesn't seem fair or equitable!!! NO ONE WHO WORKS FOR A LIVING SHOULD HAVE TO LIVE IN POVERTY!!! ATTENTION [Government and Employers*]