Plus, Andrea Bocelli works about six to eight inches from the mic. Given the choice I would start every opera singer as far away from the mic as possible. But opera singers have a great need to hear themselves. In opera houses, they use the reflective surfaces of the halls. When you take them out of the opera house and into the arena, all of them need something to help them hear.
Most opera singers learn to work with sound systems pretty quickly.
Some can just use what they hear from the edges of the system without any monitors, and others need a lot of foldback. Additionally, most tenors who've had sopranos singing duets in their ears all these years lose some hearing as they get older. So the need for monitors increases as that happens. However, they all learn very quickly that the closer they get to the mic the louder the monitors get, so they end up controlling their own monitor volume that way. When they get close, you start hearing a lot of mechanical sounds you wouldn't hear otherwise, such as breath noises.