I won't pretend to be an expert---today being the first day I've read about it---but, the very first Wikipedia article I read about the topic discusses qualified immunity w.r.t. "government officials", not "officers", which is a much bigger category of people. [1]
Police officers just stand out because their actions can have much bigger consequences and they often act as individuals. But I don't think it should mean QI cannot exist, just that there should definitely be a different standard when it comes to life and death (or freedom in case of arrests).
Because it's useful to have the enforcers of state power to be above the law.
It's a bigger problem for police officers than the clerk at the DMV, because one of them walks around with a gun, a nightstick, and a pair of handcuffs, and extremely broad discretion for their use.