Olympic boxers usually wear a tank top, but pro boxers never wear shirts in the ring - why?
There are many reasons for this.
Boxers use oil/vaseline to make their bodies more slippery. Obviously doesn't work in a tank-top.
12-rounds of boxing against 8/10oz gloves would end up wrecking an olympic style tank top really fast. Something would go loose, and there would have to be breaks in the fight. You don't want to hit anything that is not just the skin for this reason.
Scoring. With a perhaps little loose tank top, judges can't see if a body punch connects or not. Scoring is already pretty questionable in boxing; there's no reason to add additional x-factors to it. Pro fighters can be insanely fast;this could potentially be game-changing.
I think this is just plain history. Rather than pro boxing taking the shirt "off", amateur boxing put the shirt "on". Pro's just kept doing what was done for the past 100+ years.