I might really want to invest in you, so I'll blow the valuation up. I might even be the only person willing to pay that price. But that doesn't mean I want to buy you outright at the valuation.
In practice that doesn't happen. In these late-stage rounds, there is not usually a single investor willing to pay way more than other investors. And even if there were, the company would think twice before selling at that price, because it would just set them up for their next round or IPO to be a down round, which would not be good.
