why attributes is Non-reactive object?

[tangjian1891]

i noticed that attrs is a proxy object.
docs https://vuejs.org/api/composition-api-setup.html#setup-context. "attrs and slots are stateful objects".
i don't know what is the difference between reactive object and stateful object.
i can't find them in the "Glossary"

---

#8421 Using isReactive(attrs) returns false,but attrs in watch or computed is correct.

i am very confused. why is it designed this way?