Basically, manga reading, and therefore anime watching, in Japan pretty much permeates throughout Japanese society, i.e. everyone consumes it from when they are kids, to well into adulthood. (see Jiisan to Baasan last season)

Therefore different manga and anime that appeal to multiple interests, ages, and demographics are written and produced.

In the West, it’s largely the domain of young people, mostly males, but a strong female contingent in the audience too.

These, I feel, are used to be largely introduced through anime, as opposed to manga.

Although the availability and popularity of translated webcomics, manga and light novels/web novels has exploded in the age of smartphones and social media, so Western audience demographics are skewing younger than ever now.