The reason is because the subject is always 'YOU'.

The imperative sentence is usually used when one person is commanding another person. Or Instructing another person to do something. It might have an exclamation mark if the command is strong. Example: Clean the floor! It might not have an exclamation mark if the command is an instruction. Example: Consider the alternative. Imperative statements usually do not include the subject to whom the person is speaking for the same reason Korean does not usually include "I" or "You", "He" or "She". 왜요? Because to whom the imperative statement is referring to (or spoken to) is obvious. However, "You" is sometimes said just before an imperative statement when the need to get the subject's attention is required. Think of a military commander yelling at his troops and one won't stop talking...he might say... Example: YOU! Stop talking! (to indicate to whom he is commanding) 이해했어니?

I guess the reason is that this kind of expression is more polite.