Its limited because they are using low power LPDDR3 ram, which its limited to 16...

ihuman on June 13, 2017 | parent | context | favorite | on: Tim Cook Says Apple Is Focusing on an Autonomous ...

Its limited because they are using low power LPDDR3 ram, which its limited to 16GB. LPDDR4 doesn't have this limit, but isn't supported by intel on Kabylake.

https://9to5mac.com/2016/11/01/macbook-pro-16gb-32gb-ram/