Because emoji are tacky. You want to use them in your commit message? Fine. But if I were ever contribute to a project that enforces such a rule I would start every commit with the middle finger emoji.
Isn't tackiness heavily context, culture, and timeframe dependent? Besides, computing is far too serious these days . I see no reason for my computing to be a little bit whimsical (especially if it's a hobby project), provided it's also self-consistent.
I believe the opposite: that there is sometimes the right place, the right project/people, and the right time to be whimsical. So not always, but also not never (as an abolitionist stance would see it).