"Ive been told most pro boxers dont snap there punches, WHY?"
Because most pro boxers aren't very good, and will end their careers as mere stepping stones for the boxers that DO snap their punches.
Some other relevant points:
1: A few rare HW's got away with slower puches because they had so much muscle & weight behind them. A prime example is George Foreman.
2: Some great boxers throw SOME thudders to the head, but this is to confuse the opponent. If evey shot you throw is the same speed, no matter how fast, a great opponent can start to time & avoid them. By mixing them up, you catch him off-guard with the really fast ones. Prime example: WLad's left hand.
3: Some great boxers do throw more thudders than other boxers, simply because they go to the body more. Great body punches thud, because the body cannot bounce back like the head can. A body punch needs power and leverage to be effective, whereas a haed shot (against a slick opponent) needs speed / acclleration.
4: Thudding punches really DO use up more energy than fast snappers. This has been proven many times in scientific tests. It's not a theory.
5: Very rarely, I actually know what I'm talking about. This is one of those times.