I find it amusing just how many people make this assumption up-front. Probably because its one of those "problems" that seems simple on the surface, until you start digging deeper. This gets especially true once you take into account offline delivery, presence management, delivered/read receipts, group/broadcast use cases, efficient use of the network, complexities of reliable/efficient cross-platform media transfer across a variety of formats, and... robust end-to-end encryption.