People do use them, though. Look through the fastjson dependents on GitHub[1] and you'll find serious projects from the Kubernetes special interest groups and various blockchains. Heck; elastic even built their own for beats[2]. There's no big philosophy difference here. Most go programmers choose encoding/json over fastjson for the same reason most Jackson programmers choose databind over streaming: it's a simpler API.

[1]: https://github.com/valyala/fastjson/network/dependents?depen... [2]: https://github.com/elastic/beats