I have also used genetic code to repair my epidermis with a fibrous tissue after injury. It keeps moisture in and pathogens out.

To a certain extent. Often times the regenerated "tails" are fatty stubs.

Future humans will be part human, part lizard, part zebra fish, and seveal other species. We will regrow teeth, heart, and limbs. Sometimes we might accidentally grow a tail.

Because nothing comes without a cost. Maybe mutations that would allow us to regrow limbs also lead to horrific cancers that snuffed out any chance for reproduction. Maybe the benefit didn't outweigh the additional energy budget. Maybe it was just not necessary for procreation vs our advanced cognition so it never played into evolutionary fitness (compared to a lizard that can drop a tail when it's attacked, and regrow it for the neYt attack, which would surely improve its evolutionary fitness).

Additional reasons: our mammalian organs are highly complex, we live a long time, we are large animals with giant brains and high metabolic cost. The list goes on.