Because people walk on the ground on two legs, and do not run on four (then the tail can come in handy to steer at high speed) and do not jump on branches (the tail can cling to branches or, again, steer in a jump).
And about the rudimentary tail - the tailbone, you probably heard back in school, this is hello from Darwin's grandfather and our distant tailed relatives.
In the early stages of development, the human embryo has a noticeable tail, but during embryo (early period of individual development), the parts of the embryo surrounding the tail overtake it in growth, and accordingly the tail ceases to protrude and be noticeable. Let's remember the history. Initially, all living things lived in the sea and the tail was an organ of movement. When the organisms reached land, the tail became not only an organ of movement, but also an organ of balance (like in a crocodile), a place for storing nutrients (in lizards) and even a rudder during flight (in birds). In humans, almost all functions of the tail are replaced by hands. There is no need to fly, we maintain balance, standing on two legs, respectively, and we do not need a tail. He will only hinder us. We have a small rudiment left from the tail, called the coccyx. In our world, of course, not without deviations. There is such a phenomenon as atavism, the appearance of signs characteristic of distant ancestors. If the fetus does not develop properly, then a tailed person is born. Usually, a child with this phenomenon is immediately operated on. Operation without complications