Sorry for off-topic, but: why are there many different neurotransmitters instead of a common single one? (I mean, sure, evolution, there's no "why", but I'm asking from an hypothetic engineering standpoint.)
There are a bunch of different aspects to being awake. Caffeine works on adenosine, which has to do with wakefulness. But also things like adrenaline have a different aspect of wakefulness that amphetamines target. Modafinil targets orexin receptors I believe, which involve wakefulness as well as eating.