It's almost certainly about power consumption; Apple absolutely loves being able to tout huge battery life compared to the competition, and one way they do that i s by being tighter on things like RAM usage.
It's also worth pointing out that the RAM lives in a Package-on-Package configuration right on the CPU die. They're not exactly buying mass market SO-DIMMs and sticking them in a bog-standard bus, so the cost per MB is probably higher.
It's also worth pointing out that the RAM lives in a Package-on-Package configuration right on the CPU die. They're not exactly buying mass market SO-DIMMs and sticking them in a bog-standard bus, so the cost per MB is probably higher.