Skynet has been defeated alot

 Skynet has been defeated many times such as the human resistance destroying its main core in 2029. It  lost during the future war when John Connor and Kyle Reese fought together and its  resistance soldiers destroyed it there. The only option that skynet did to try and get back in the game is it sent one of it's terminators through a time displacement field and to try and kill John Connor before he was born. No matter what terminator skynet brings, the terminators never seem to accomplish there mission. In alot of different timelines skynet has been destroyed. Skynet could've won against humans pretty easily but I guess it felt guilty of destroying billions of people so in order for it to survive it puts humans in there skynet work camps to be used as slaves and batteries. We beat skynet because we are unpredictable and clever such as scavenging for supplies and weapons. Using their own machines, ideas, and future tech against them to win the war.