Email list cleaning is an important factor, as it helps improve your email deliverability rate, open rate, and IP reputation.
There could be a possibility of human error while entering the email address. Also, if the email database is quite old, there might be the possibility of dead emails in it. It is also possible that any subscriber has an opt-out from the email list.
Due to these several reasons, your email gets a hard bounce, and continuing this practice, unknowingly can end you on a blacklist.