Japanese anime and manga has developed its own visual language or iconography for expressing emotion and other internal character states. This drawing style is very distinctive with unique facial features and character traits which are very very different from real life and explaining each of them elaborately will make the answer very long. So, let's stick to only "mouth" as per the question. Mouths are often depicted as small, usually rendered with one line on the face.
So, why single line?
This is kind of a generalized explanation. There are several factors come into play:
- Tangible economic reasons: Animation is shockingly labor intensive and expensive, meaning if there's a way to skip some detail without alienating the audience, animators will go for it. So, not drawing defined lips is a very real time and money saver for the studios. Even if you move tiny amount of pixel, the whole emotion of the character changes saving a lot of time for the animators.
No Makeup: Female makeup on average is significantly less pronounced in Japan (but that doesn't mean that Japanese women or girls wouldn't wear any). But the general preference leans strongly towards subdued, non-eye-catching hues, even for actresses or models and even for younger women/girls in everyday settings (many school regulations straight-out forbid lipstick entirely). This of course had an effect on anime character designs in turn, after all a big part of female anime characters is supposed to be in or close to that age range. So, in anime as soon as you can see any hint of the lips at all, it's usually supposed to mean that the character is either wearing fairly conspicuous lip gloss - or even colored lipstick.
Psychological advantage: our brains are biologically hard-wired to subconsciously categorize certain physical features as non-threatening, and will inevitably respond with feelings of affection or protectiveness. So "big eyes, small mouth" is a part of this. See moe-culture for more information.
So, bottom line is these types of faces are not typical Asian-looking but are so popular and visually appealing to viewers that they recycle it. Making realistic faces in anime is somewhat expensive. Animators pertains to a distinct anime art style (which is different for different studious). This distinction is very important and if every studio tries to make real life faces, then the distinction would be lost and it won't be visually appealing to viewers. That's why they intentionally let the facial features quite different from real life and also from other studies. (see this video to see different art styles of studios).
- Why does anime characters have no lips? -Quora post
- The DISTURBING Truth Behind Why Anime Characters Don’t Look Japanese
- Anime Faces how real are they?