All successful MMOs contain a good mix of Player V Player and Player V Enemy content. Zwift is a cycling MMO right? yet it has no PVE content. The market for gamers who would like to get fit is massively bigger than the market for serious cycle trainers, so why not. On top of that, for those of us who use it for half hour/45 minute lunchtime training, full on races are never going to be a thing.
What Zwift really needs, to broaden its audience, is ghost races and bot races of customisable duration, distance and difficulty.