Same reason she cheated on her boyfriend and denied about using his credit card.
Yes. Agree. But in this case it was primarily because McClean helped her write the letter! McClean, the ex-FBI agent, is probably complicit in this conspiracy to take Kavanaugh down. When Ford referred to advice from her “Beach friends” it probably was ex FBI agent McClean in Rehheboth DE, not some surfer airhead types in Malibu!
I'm guessing that boyfriend is not the only person she has cheated on, defrauded, or lied to during her life. I'm also guessing that a bunch of people are going to be questioned about her, and she will not be pleased with what they say.