Why does the dentist make you have more than one appointment for a single cavity?

I would like to preface by saying I don’t mind it, I’m just genuinely interested in the reason behind it.

I have a cavity and I have to go back 3 times to get it taken care of. When I asked if they could do it all in one day, I was told they could but the likelihood of the cavity returning would be higher. Is this true? I’m from the U.S. and as a kid I remember them just doing it all in one appointment.