Humans have a vestigial tail because it is an essential part of forming the spine. The spine's growth is a step that is a part of embryogenesis every embryo must go through to grow an entire functioning system, like the spinal cord and skeleton, it cannot be totally rearranged because we no longer need the post anal portion of the tail.
New developmental controls are added to overwrite the previous developmental progression after the critical point of producing a functional spine is passed. This produces a new form, like a coccyx, that is modified from the earlier tail-form by adding genetic regulation. It involves a new coreography of development.
In early embryonic development as the neural cells migrate into place and are enclosed by the vertebrae the full tail is formed. At this time all the spinal cord's neural connections to the limbs are routed. This is why month old human embryos clearly have full-length tails.
Once the full spinal function is developed the distal coccyx is modified from the earlier tail. This triggers a strict replacement schedule (or programmed cell death called apoptosis) to accomplish this remodeling. At this time the fifth through twelfth coccygeal vertebrae are removed by activation of the coccyx remodeling regulation. A normal coccyx retains only 4 fused coccygeal vertebrae.
In short, after week 5 the fetal tail is shortened by apoptosis & absorption, then the remaining vertebrae are fused. This way the fused coccyx is still able to support any residual but essential uses in stabilizing limbs with the pelvic girdle.
Vestigial structures cannot be completely eliminated because eliminating their growth alters the growth of other organs. The larger structure can be reduced or reshaped only after the point where disruption is no longer developmentally lethal to the growing embryo. Cacti no longer grow leaves but cannot eliminate the meristem leaf primordial cells without disrupting other organ growth from meristem.
Cacti have vestigial leaves reduced to microscopic proportions. This has freed the meristem to be adapted to form spines rather than leaves as in other organisms. Growing spines is a useful redirection to the ability to grow leaves from these meristem tissues for a xeric habitat.