I wonder when someone at apple will clarify that it's because there isn't room on the motherboard...
The response about battery life is really silly.
DDR3 CHIPS HAVE MAX SIZE OF 1GB EACH.
Have a look at the tightly packed motherboards apple makes from the an
iFixit teardown of last gen 15 retina (yellow box).
RAM is already the BIGGEST space hog on the motherboard, nothing takes up more room. No one is soldering more than 16gb onto a motherboard. They would need to use height to stack in more memory if they wanted over 16.
DDR4 page on wikipedia mentions 2gb chips are in the spec, if I read right... But even if Apple went DDR4 tomorrow I don't think anyone is manufacturing 2gb DDR4 chips yet.
I think 32gb soldered on the MB is still 2 years away.