Most people barely pay attention to the world around them.
This.
Or, if someone sees you open carrying and you're even remotely in shape and clean cut, they assume you're a cop.
I have a sort-of open carry story...
I live less than three miles away from the Shirley Rod And Gun Club. Many times when I'm working up a load, I'll load a few rounds, put them in my pocket, put a pistol in another pocket, and drive to the range for a quick function check.
One day I loaded some .45 ACP rounds that I wanted to function check in several guns. I didn't feel like loading up a range bag or locking everything up for what was going to be a 10 minute visit to the range, so I went there "Doobie style" - I had a RRA 1911 in a Fobus holster on my left hip, a Colt in an old retention holster (backwards) on my right hip, a S&W 4506 in my left cargo pocket of my shorts, and a Glock 30 in the right pocket - all loaded.
It was hot as hell and I was thirsty, so on the way to the range I stopped in at the little general store in West Groton to get some Gatorade. I pulled my T-shirt down over the exposed holsters to barely cover just the tops of the 1911s on my hips. It looked ridiculous. There was a uniformed cop in the store investigating a suicide at the old age home across the street. He asked me if I'd seen anything unusual, we chatted for a moment, then I paid for my drink and left. If he noticed, he didn't bat an eye. I don't think he noticed.