Why do historians have to include a citation when paraphrasing an idea from another source

It is 1902 i make 45,000 per year how much do i owe in federal income taxes

The declaration of independence gave the americans concrete reasons for the revolution

Is karma the same as destiny, in the sense that everything that happens to you is predetermined? do you believe that "what goes around comes around," and if so is this the same as karma? does everyone get what they deserve, in the end?