Because it's being questioned, and she has yet to produce a shred of proof (pay stubs, w-2, tax filings, pic of her in uniform on her first day, etc), combined with the fact not a single person has came out and said, 'this is true, I worked with her/was her boss'.
Pretty sure if someone I worked with at 19 was the vice president I would remember working with them, and any former co-worker that is a Dem, would definitely come out to shut up the Republicans saying she didn't.
Why can't liberals use common sense? 🙄