Photoshop: Why does resizing a document without a background layer causes transparent pixels on edges?

Yes, and that is entirely expected. Outside of your layer there are transparent pixels. When you resample, those transparent pixels get mixed with the pixels on your layer. This is a natural result of the math of resampling and that fact that Photoshop allows layer data outside of the document bounds.

As for intelligence: Photoshop is not all knowing. It can't guess at what you intend, and it really should not treat pixels at one location different from pixels at another location. If you had color pixels beyond the document bounds - then your suggestion would resample them incorrectly, and would lose data when the image was resized.

And the same "problem" can occur for shapes that don't cover the entire canvas, but exist on layers smaller than the canvas.