There are multiple reasons.
- They do not want to fight to begin with. Why are they even there?
- They are going to go hard when they get into it. When you match them they're gonna blame you for hitting too hard
- They belong in a dance club not a martial art dojo. THERE IS NO MERCY IN THE DOJO
Maybe there are more but that's all I can think of right now.
Yeeesh. This is a troll post.^
But for the sake of other readersvhere....
I get that some newcomers to the gym have some real ignorance and they don't understand themselves in the gym at all. That's a self discovery process. I get it. But also, I've had very high level fighters let the coach know they want to work on specific things through sparring so the coach creates that necessary sparring "he's working on ______ so make him work for it."
Thai fighters at the highest level understand the value of learning good habits through light or 20% sparring whatever %. There are times to spar hard, and there are times to spar light. Each serves its purpose. Hard sparring is really valuable to get you used to the intensity pressure and punishment of an actual match. If yoy only hard spar you end up with bad habits and "survival mechanisms". Sparring at different intensities let's you get better at seeing what is happening.
All good progress and change takes time. Hammering away with harder sparring makes one more brittle than better.
The only point you made that I can understand is 2. There have been people who ask to go light and then don't realize they are intensifying the sparring and then they don't understand why you match that intensity. The answer, to me, is to have coaches present to have eyes in the sparring. People training can have egos (sometimes) and they don't like hearing feedback or advice from people they spar. But the coach should be there to say: "you did this, so he did that."
I sparred without coaches, but that was with people who I knew from training and we both knew that if it intensified it was because one of us caused it and we were preparing for our own matches so we recognized we were there to help each other. Mostly though, sparring with coach present is the best way to do it. Some gyms have assholes in them and will be foolish people to spar with without supervision.
Sparring is learning. Also, everyone should respect the pecking order in the gym while still having dignity and trying your best.