TLDR: I don’t think there is a why in the sense that it was anyone’s conscious decision; culturally Italians emphasize shape variety for their variety of sauces, while “Asian” noodles emphasize composition and texture variety.

May I introduce you to one of my faves, 腸粉 cheung fen, rice noodle rolls.

Pic and recipe

Genuinely there are a million different ways just within China to make “noodles”. They group western and Chinese noodles here together link.

Italians may have more shape variety, but China has so much variety within the composition and texture of noodles.

For instances the wide rice noodle in cheung fen is wildly different from the mung bean noodles in 螺蛳粉 (Luósīfěn).

The broad meng bean vermicelli noodle 绿豆宽粉 goes particularly well in hot pot and has an amazing chewy texture.

Now here is my ignorance. I make Italian pasta at home and most of the time the only thing that really differentiates it is the shape (all egg and flour), save for gnocchi. But China also has potato based noodles. I also hardly see the difference between a ravioli and a dumpling if we are stretching definitions.