In answer to the OP’s question I think a lot of it is because they find it inconvenient. When it comes to helping people you will never be on the right end of it. I used to be one who always stopped if I thought somebody needed help; but met too many ungrateful people and had a couple of scary situations. One day on campus it was just starting to snow when I saw a woman come around a corner on a moped and completely wipe out. I ran over, pulled it off her and asked if she was all right. Her reply was so full of vitriole that I almost picked the bike up and threw it back on top of her. I came across a mother and son sitting in a parking lot with a flat tire, long after the store had closed; he was about my age. (Early 20s) I offered to help so she gave me her keys through the window and I opened the trunk, got the jack and spare out, tried to loosen the lug nuts and broke their 4way wrench. I went to her window, explained what had happened and said that I was going to my house a couple of miles away to grab my breaker bar and sockets. I did so, got their tire changed and put everything back into their trunk. In all of that time neither of them got out of their vehicle… nor did they thank me. I went out of my way for them that night, and all they did was sit there. It really left a bad taste in my mouth.
In the back of my mind was that they might expect me to pay for the tool that I broke.