Because the guy in the street is performing the role of the perfect Uke. He is giving an honest, committed attack. He's not trying to resist, he's trying to punch a face. What more could you ask from him as far as giving you something to harmonize with?
How many times have you been in a real fight? What you are describing is how most people in aikido imagine
a real attack to be like since it, not coincidentally, precisely matches the fake attacks they teach people to do in the dojo because that is what they need to make their techniques work.
It's very common for people to assume that harmony in aikido means that people should or will harmonize with them and meet their expectations; then when that doesn't happen they get upset and start reverting to the same childish behavior you see on the playground from ten year-olds (see another current thread for examples of this). I believe that it means that the person doing aikido should adapt to and harmonize with whatever the other person does, including resisting your attempts to throw him, which is what any sensible person would and should do.