because like most jobs you learn the material to get hired, but then forget or just stick to what you need to know when you get in a routine. for a cop all they need to know is that they have a badge and worst case scenario if someone kills them that person will go to jail forever and their family will get targeted by police.
but fr most cops are former high school athletes who's sports career didn't pan out and they weren't smart enough to finish college without the tutors and other shady shit, so they figured they can make a decent living while also maintaining their power as a cop. most of them are used to parking on the side of the road handing out traffic tickets that they are feening for the day that they can pull out their gun. then nerves take over and they forget about their duty and it becomes "my life or their life" even if the situation doesn't call for that reaction (like this one and countless others).