>why do we say "la película es buena" but "el día está bueno"?

I don't know if I'm quite qualified to lecture you on your mother tongue, but AFAIR that's because one is an unchangeable inherent quality (of the movie), and the other one is a temporary condition (of the weather).

As a learner, I find Spanish/Castellano to be quite consistent, actually.

It's normal that one's native language's quirks tend to be very difficult to explain, because you learned them intuitively. E.g. I was surprised to learn from conversations that a lot of Spanish speakers seem to have never heard what the subjuntivo is, and being told, couldn't explain for the life of them why they use it all the time.