Because if nobody complains, things don't get better.
In the situation I complained about, there was an apparent system error which meant an item was being overcharged at the tills by, if memory serves, about £12.
Plus I'm in my mid seventies and not in particularly good health, and was asked to walk back right across a very crowded store to take a picture of the price label on the shelf to prove to the assistant that I was being overcharged.
I'm very sorry if you think I was complaining about nothing. I don't.
I wonder how many people were overcharged that day by £12 and didn't notice because it was Christmas Eve, they were stressed with doing last minute shopping, and had so much in their trolleys that they didn't have time to notice the price being charged for each item on the till.