many reasons why not but here's one - the capacity of blood and air needed by a baby is far far less than a human adult so if you attached yourself back to your mum you would kill her pretty quickly through strangulation or blood loss.
No. afaik in an adult the belly-button is only a 'scar' with no attached bloodvessels for lifesupport. These start to be reabsorbed by the body as soon as one is born.