Because if time is not a loop, then there must be a beginning of time, and a beginning of time can't exist, because if so, what is before time? Nothing could have caused the beginning of time, because that nothing can happen without time. If it was a loop, time would cause time to begin.
Any point in a loop can be taken as a beginning. You can make a loop out of line and a line looked from the top looks like a dot. What I'm saying is that depends on the perspective. Logic is not good here. Also, do trees understand time? Are we born at birth or with the first memory?