It's not "because they want to do another round." They don't necessarily want to give you the extra money, but the final amount of their return does need to be a multiple of the number they could have invested, not the number they did invest.

Thus they need your company to grow more than you might think they do, and they could block a sale in the hopes that will happen. If you end up needing more investment after the sale has been blocked, that's fine, because they have money in reserve, but actually giving you the money is not the overwhelming motivation.