I don't have a link but read that in an interview they said something to the effect of the lack of in-game communication was intentional. Their rationale was that there are a wide variety of chat apps, with different corners of the globe having popular preferences that may not be the same in other areas of the planet. They said they didn't want to force users into using a chat system that may not be as preferable to their existing one.
Personally I think that response is a bunch of company fluff to hide their laziness or unwillingness to develop one behind. Some method of communicating with other trainers in-game could make for a better experience. One of the concerns people have is that this could lead to toxic communities to become worse. I think if chat was kept to pre-defined messages it could mitigate this.