For the Road Warriors: Why are Burnouts in Drive a Bad Idea?

I had the same transmission for about 10 years from the time I was 18 through 28. Needless to say that it saw countless burnouts but when a friend of mine rebuilt it a few weeks ago everything looked brand new. I never let the rpm go about 4000 in 1st before shifting to second though. BTW these burnouts were with street tires and slicks. I beat the snot out of that car and the only mechanical failure was a spun bearing.