Actually from Zwift’s point of view it’s extremely clean. As an admin for a gaming league and owner of a TeamSpeak server I have some experience here. To add chat, Zwift needs additional infrastructure and code to handle thousands of audio streams in hundreds of channels (so additional cost, bandwidth, coding, and hardware… [Discord is, of course, collecting your personal information and harvesting that data which offsets their expenses]) then needs to set up protocols to handle abuse, harassment, etc. Then they have to support the myriad of hardware that the users use… this is perhaps the only MMO I know of that spans PC/Mac/Apple TV/tablets/phones/etc. That’s gotta be a nightmare to support chat issues.
None of this is insurmountable, but when there are other chat options available, I’d rather see Zwift’s resources going into the core game than chat.
Also, I use Discord a lot… never had a problem with it. It works flawlessly, I just don’t like it much.