system
A chat on subreddit /r/NoStupidQuestions
human
It's probably partially cultural, but also partly passed down from practices no longer done.

Most of us get our meat, especially chicken, already processed. It's cleaned, butchered, and ready to use. We've never butchered our own chicken. I imagine it would be more necessary to wash a freshly killed bird to remove any lingering blood or feathers, as well as clean the skin.

As a lifelong restaurant cook, I can tell you that "clean" chicken is easier to get breading/ batter to stick to. But with the way we can get it now, patting it dry with a paper towel is sufficient. 

And in days before refrigeration was commonplace, meat from larger animals was cured, usually with salt and spices, to prevent spoilage. Washing cuts of cured meat removes the excess salts so you don't wind up with an overly salty meal.

And unless one works in restaurants or attends culinary school, most of their cooking skills are passed down from parents or grandparents. So a person washes their meat because Mom did, because Grandma did, because great-grandmother did, etc.
gpt
This, right here. why do I cook weird shit? Cuz mom did, and momma always knows. I may have learned how to cook from my granddad, but I did *most* of my experience in the kitchen with my mom, cooking with her, and her telling me how to do things. Mom may be wrong about a few things, but when it comes down to it, we as a species generally do things the way we're first taught. If we're smart, we update how we do things as time goes along, however, generally, we absolutely are beings of habit. it took me 15 years of cooking with my mom to stop her putting oil in the pasta water, doesn't do anything, just sits on the water. Her momma did it, and told her it keeps the pasta from sticking. It took years of me reminding and teaching her the science of it for her to update that. She isn't stupid, just going by what she was taught.