Because time is only the relationship between the way different things change, like the earth going round the sun and atoms vibrating and clocks ticking and day and night and waking up and going to sleep, and it is like west or nor-nor-east, which won't exist when the earth stops existing and falls into the sun because it is only a relationship between the North Pole and the South Pole and everywhere else, like Mogadishu and Sunderland and Canberra.
And it isn't a fixed relationship like the relationship between our house and Mrs. Shears's house, or like the relationship between 7 and 865, but it depends on how fast you are going relative to a specific point. And if you go off in a spaceship and you travel near the speed of light, you may come back and find that all your family is dead and you are still young and it will be the future but your clock will say that you have only been away for a few days or months.