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?

Because a lot of times guys don't listen when a girl says no I'm not interested and only choose to leave her alone if they know she's dating someone. Though some guys don't even care about that and end up getting offended about being rejected regardless of what you tell them.

I've personally never done it but those are the reasons I've heard.