Why do most anime come from manga ?

Because they want to make sure their anime production able to gain revenues or profits from popularity of manga. Making an animations is not a cheap thing.

So their business mindset is if the manga very popular and selling well, so their Anime production may have same chance as well.

So not all manga have same chance to be make into their anime version. Some manga may still remained in manga version because less popularity, bad sales or other reasons.

Same goes with Anime figures and merchandises, their productions also depending with both niches popularity as well.

It's working similar like Domino effect.

Why American comics not as popular as Japanese comics or anime series ?

 American comic has their own niche and strong fanbase different with Japanese manga.

That's the reason why American comic example like Marvel and DC still exist since past time until today. Recent Marvel and DC heroes movies also helping their comic popularity and increasing their own fanatic fans as well.

But the bad thing about American comics is they more less variations genres than Japanese comics. Most of popular American comics always related with superhero theme.

About superhero theme comic, Japan also following American comic example. But American comics still hold the crown by their giant franchises and strong fanatic bases around the world.

Disney comics also very popular as well and known among their fans, example like Carl Barks Scrooge McDuck comics.

My Hero Academia