Plus even the best singers pick up bad habits they might not notice.
I had formal lessons for years and now that I don’t I’m certain some old and new habits have cropped up. It’s smart to take some lessons time to time to get fresh ears on your technique and suggestions for improvement. Even the best singers DO have pitch problems. They might be minuscule, but they do occur. Sometimes the singer hears it themselves but can’t figure out how to fix it. A teacher can point out where this is occurring and offer methods that will strengthen pitch.
Some professionals and amateurs do take ongoing lessons. These are expensive. My teacher was $55 an hour and she was cheap for the area. Most classical training is closer to $75-100 an hour, or more. If you go once a week it adds up! So those who want to make singing their profession do hope to be paid as a cantor. I know several professional singers who cantor as part of their career. Usually they are also teaching, singing here and there for events, preforming in theatre, opera, or classical concerts, and often they have a “side job I have to also do to make enough to survive”.
As for me, this is not my career, I’m just happy to cantor. So I gladly give what I can. I sound professional (I guess), but I am not. I’m grateful to be participate in Mass in such a way. However, I wouldn’t begrudge a cantor for being paid. Chances are they are just trying to make a living doing something they love. They may be very grateful that their gift allows them to both glorify God and contribute to their means of living. Plus these professionals can be called in to cantor for weddings and funerals, which may call for someone more polished.
As per rehearsing, a cantor, professional or amateur, may spend hours practicing each weeks music. If they are to lead the congregation in song they have to know all the hymns as well as the psalm, sung responses, and anything else that might be asked. There is a great deal of preparation. It takes time. For the hour or so long Mass your cantor may have spent the entire week ahead going over every note and even rehearsing flow. She or he may be carrying this music with them to practice wherever they go.
Your cantor, assuming they are Catholic, might even attend two Masses that weekend because cantoring takes so much out of them that they might like to celebrate Mass more freely without the responsibilities.