Please explain why are we not supposed to swallow toothpaste?

It's okay. It's okay to swallow toothpaste. We just tell children not to swallow it so they don't get too much Fluoride that can cause some minor discolorations in the enamel.
Toothpaste. That recommendation is primarily for children to make sure that they don't over consume fluoride as teeth are forming. If you have a child, discuss appropriate toothpaste portion with child's Dentist or Pedodontist. As an adult, you would have to swallow a bathtub full every day to even begin to see a problem.
Ingredients. Toothpaste is not formulated to be ingested. It is designed to be used in the mouth, and then spit out. Ingestion of those ingredients can upset your stomach.