Why is there initial margin when I did not borrow any money to begin with?

Although "margin" is often used colloquially to mean borrowing, technically margin is sort of the opposite: the amount you are required to put up yourself instead of borrowing. It makes sense because what you put up yourself is the "margin of safety" for the broker (the amount your investments can lose before the broker's own funds are at risk).

Here the broker is telling you that from your 895k account you could withdraw in cash (without selling any stocks) up to 608k! This would leave your "skin" in the account at the minimum level of 287k. If you did this and your stocks declined any further, your account would be called/liquidated to protect the broker.