why do girls/women lie and say they have a boyfriend why not just tell the truth and tell the guy that you're aren't interested or we can be friends?

Let me give you an example that is far too common. A woman is sitting at a bar with a few of her friends, having drinks. A guy has been eyeing her across the room and approaches, asking to buy her a drink, clearly interested. If she says "No thank you, I have a boyfriend", he will likely walk off. If she says "No, I'm not interested" he might ask her why and continue to question her about it, making her uncomfortable. He might insult her. He might try something anyway. The sad fact is there are men out there who do not respect a woman saying no because she doesn't like them and will only respect a woman saying no because another man is involved in her life.