Is it healthy to leave a dead/root canaled tooth in your mouth for life? if any other dying tissue is removed from body? why not dead teeth?

Yes: A tooth that has had a root canal has had all of the necrotic tissue removed and the canal is cleaned and sealed. In most cases, this will prevent any further bacteria from reinfecting the tooth. It is very important to have a permanent restoration on the tooth after a root canal to prevent recurring decay and seal the tooth at the access opening as well. This may be a crown or filling.

Tooth not dead.: The tooth is not dead after a root canal. The nerve and pulp tissue within the tooth has been removed after a root canal. The hollow spaced that remains has been sealed from the rest of the body. You can still tell when you have something between your teeth because the supporting tissues of the tooth remain between the tooth and the bone.