Other answers have commented on how many players do get hurt, but I'll say if you have a large amount of sweat on your skin (like when you've been running your butt off in a soccer game) can act as a pretty good lubricant. read more
Football players do hurt their knees but they slide tackle anyway. And slide tackling has easily got to be the second best thing to do in the sport. It's addictive and fun and extremely useful. read more