Because there's too much not shown or explained even the chars inside the movie don't understand how things are working. Like Steve materializes on a park bench (or shows up?) and Prof. hulk who helped make the time machine doesn't seem to know how he did that. He's like "I'll get him back! I'll get him back!...Oh there he is" It's like what???? A lot of the explanations are just conjecture,there's no real answer.