There are many reasons one may resist arrest. In the academy we were taught that a subject may resist simply due to shock, which is why communication is important. If someone catches you off guard, and escalates things by physically controlling you without communication, its easy for that person to be in shock and not actually comprehend what is going on, or to follow orders. Officers are supposed to deescalate a situation when possible, not the opposite. My CO told me in every confrontation I would be escalating it, because even though they may not be armed, I am. Its my job to turn things down a notch and explain whats going on, why I stopped you, why I am arresting you etc... not grab your arms and throw you onto a car hood. You will notice if you watch real cop videos, that those cops who take the time to explain things have a much better compliance rate, and study after study has backed that up.
Further, one may resist due to fear. Fear that they may be shot simply by cooperating with the officer and getting their license out, which HAS happened several times, including the incident in South Carolina where the officer was found guilty.
And honestly, I am not sure what to say to someone who thinks people risk their lives, risk much longer prison sentences and risk bodily injury just so people can protest. These are just people caught up in their daily lives, I cant for a second believe that Mr. Sterling thought "wait a minute, instead of complying with these officers who are threatening to shoot me, Im just going to keep squirming and risk being shot so they can protest on my behalf". I mean come on, I hope you really dont believe that?