Despite 2D animation being sequential still drawings and being a drawn medium such as illustration and comics, it uses the fundamentals of drawing such as line, form, and shape. It also uses drawing techniques like shading, perspective, and foreshortening, and line weight such as thin and thick lines. Hatching has been used in various art forms since, but why do 70s anime use hatching so much? Is there a reason behind it?
Hmmmm.... maybe because it's just the Japanese artists' style.
In Japan, it's called "Kakeami".
In manga, kakeami カケアミ is a cross-hatching pattern used to shade objects and backgrounds using lines that form "webs," "nets," ami 網, which are "thrown over" each other, kake 掛け.
Hatching, you either love it or hate it as an artist. For some people, it can serve as a soothing doodle technique or become their secret weapon to a beautiful comic or manga page. What exactly is hatching?
Hatching is a drawing technique using lines or linear strokes to create value and texture throughout an image. They don't always have to be straight lines, and they don't always have to be neat and consistent either. There are multiple types of hatching, each providing their own texture that you can add. In comics and manga, it is used both as a shading technique and a texturing one. It's really very versatile!
If this doesn't answer your question, then I'm sorry. I'm only just trying to help.