What would be so bad about it? Founders already committed to other projects? Leakage of "Ideas"?
Keep in mind, we're a very early stage company. Our video reflects that, and our product video reflects the early product we have shipped (although we do hope you download and try Cuddli instead of just watching the video!). With the wide distribution and high profile that anything linked to YC is likely to attract, we may well be creating an impression with potential future partners, investors, and employees. This is likely the reason why most of the startups in this competition haven't chosen to make their video public.