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 some fellows will keep asking or trying to change her mind [b]unless[/b] she tells him there’s someone else. Or even worse, become abusive.
You’re [b]correct[/b], “No, thanks” should be enough, but for some guys it [b]isn’t[/b]. 😒