It’s hard to say why priests would do such a thing. It could be for any number of reasons.
I know for sure there have and always been men who were pushed into the priesthood by over zealous families. As were daughters made to become nuns. Some would of course go on to be wonderful religious, but then a small percentage would hold resentments.
Anger can be a contributing factor in child molestation and sexual abuse, as well as stress.
There will always be those with psychological-sexual disorders who would offend in or out of a religious institution anyway. The opportunities within a religious setting are good though, as they are in a position of trust, “supposed purity”, and authority.
There is then the priest or religious who is weak willed and has been tempted by the Devil.
It is after all the work of Satan. It’s a great way for that filthy fallen angel to destroy the beauty of God’s masterpiece of His creation, the human being. He will try as hard as he can to cause despair in the child or later in that persons life. Another reason is his continual attack on the Church from within. This is why it behoves us to always pray for our priests.
As far as numbers are concerned, no one knows really how many children have suffered or are still undergoing the abuse each year because most do not come forward. (shame, guilt, the usual things). Most sexual abuse happens within the family or by a close friend of the family and there is usually a threat imposed. An awful lot happens in other institutions, orphanages, schools, etc. as well. The Catholic Church is top of the headlines because the world consciously or unconsciously expects it to be the moral barometer.
The Church has always moved relatively slowly, but is actually doing something about it, albeit in “church time”. But there will always be those wrongly accused, some who slip through the net and some who actually are caught.
Child sexual abuse has been going on for millenia and will sadly continue under the cloak of secrecy until the end of time. Most victims taking that dark secret with them to the grave.