Because unrealistic representation of timelines can sound like a non-technical manager eager to claim credit for arduous research and mechanical work they don't seem to understand enough to give accurate forecasts for, just as much it might sound like an excited geek who can't wait for a cool thing.

Maybe it sounds like aggressive and optimistic goals. Maybe it sounds like someone who doesn't understand that nine women can't give birth to a child in one month, but cracks the whip like they could if they tried hard enough.