Investors are taking a risk with other peoples' money. And their management fees usually cover very juicy base compensation, so their downside risk is minimal.
It's irrelevant to some degree if this growth is real (driven by a better product), or inflated (by offering steep discounts, ads, marketing, etc).
It looks good on paper, and allows raising the next round. This is of course an order of magnitude bigger and demands even more growth. VCs look like geniuses in that case. Value of fund goes up, fees go up. Until they don't.