Why does twitch's stealth last so long?

It's a basic ability with a relatively low cooldown but it lasts almost as long as rengar's ultimate. Yeah, rengar ult gives him vision of the nearest enemy but in a typical solo q game you pretty much know where enemy champions are or are going to be, and even more so if your team is doing standard warding. But in a way it's better since the enemy isn't indicated when they're in danger. The duration just seems pretty forgiving for the huge value this ability gives on its 15 second cooldown with no cdr. If the duration was cut in half it would probably be just as strong for the average twitch player.