Some do, but most save it for the real fight. Sparring is practice, and is supposed to be a learning experience. You do not teach someone to fight by beating their brains out, so there has to be a happy medium. That said, the famous "Philadelphia Gym Wars" of the past were sometimes more epic than actual pro fights, and in places like Detroit and New York, the same scenario's played out to a lesser extent. All boxing towns have their stories. In general sparring is just training, and there is no need to "go all out" injury is something to be avoided during training if possible, so it is unwise to let boxers hurt each other in sparring, so most trainers keep it within acceptable bounds.

Because going all out makes injuries much more likely. If you hurt yourself sparing you never make it to a match and therefore never get to compete for money (or fame). Also, sparing is something of a learning process, if one of the fighters is better than the other (as is usually the case) its really hard to learn if your opponent gives you a concussive brain injury every time you get in the ring for a lesson.

Really it comes down to the practical fact that professionals can't go out all the time. If they did they would be so trashed that they couldn't when they needed to. This is also why MMA fighters don't end up horribly injuring each other in dojos. There's no glory to be had getting the tar beat out of you in private. It also isn't that great to win and cream the other guy if no one sees you do it.

You work on things while you spar and you work on timing and ring movement and stamina.

I never go all out sparring. I lose sparring matches to guys I could beat anytime in a real fight. It's not about winning for me, it's about getting used to taking punches(catching punches) and working on my offensive moves, timing and foot placement.

In boxing, most people don't train to win by knockout. Most people train to win by points. To win by points, you have to survive till the last few rounds, and to do that, you have to conserve your energy and only spar lightly.

Some do

I was watching some pros on YouTube sparring and I thought that when you soar your suppose to act like your in a match? Why don't guys beat on eachother in sparring? I never see pros in their sparring videos knocking out their partners. Why is this? Is sparring suppose to be light?