The Catholic Church seems to teach that we cannot ask the saints/angels for anything else other than to pray for us, but I don't understand why?

Angels have powers inherent to their nature. So why can't we just ask an angel to use his power to help us, instead of asking the angel to pray for us while we wait for God to grant the request? If it's not a sin here on Earth to ask a doctor to heal me, instead of asking him to pray to God to heal me, why can't I do something similar with an angel?