The people/investors gives the altcoin so much valuable especially if it has the potential to rise and compete with the other top altcoins. By having many investors it will give so much advantage that the altcoin may go for a long term if the developers wants to continue it.
Dogecoin is one of altcoins still increases its value without any developers, why? Do you know the reason? Because the value of an altcoin doesn't depend on the developers only, it depens on the user more than the developers, so for Dogecoin's fan, they still think this coin has the future.