That's only because we know in hindsight that it works, though. Phase 1 and 2 trials are important, even though they take time, because there's no guarantee that a vaccine candidate will be safe and effective. See, for example, a couple of Merck candidates that ended up failing [0]. If the Moderna one had been released in March 2020 and injected in millions of people and then they all got sick anyway, it would have been a disaster.
[0] https://www.nbcnews.com/health/health-news/merck-discontinue...