Anime is literally just an animation style. If you're Western, you probably grew up with certain ideas of what animation is for and what it's not. At the end of the day, fictional media is just that. Fictional media. It can be a slasher horror story, heroic knights vs. dragons, college students getting into hilarious hijinks just before graduation, two mid 30s single adults finding loving with each other, and anything and everything in between. The difference between East and West is that in the West we grew up with animated fiction being used to tell fictional stories to children, and live action fiction being for adults, so that attitude tends to carry over to how Western people look at all animation. In the East, that doesn't happen. Their cultures don't put live action vs animation into boxes like that based on who its for. They have just as much animated media that's mean for adults as they do for children. To them, it's just another way of telling a story.

Western audiences got caught in a self-reinforcing loop where they were taught Media Style A is for Audience X and Media Style B is for Audience Z. They grow up with that, so when some of them go on to be writers, directors, actors, artists, etc in media, they do the same thing since it's what they know, which teaches the next generation the same thing, and the cycled went on. We think of cartoons as being for kids because that's who they were made for when we were kids, but the adults who made those cartoons made them for us because they grew up reading Silver Age comics which were also made for kids, and so on. Look back 20 years to when Family Guy or South Park were made, or 30 years to when Simpsons was at its peak. An animated show that was adult friendly was a massive outlier, and many of those still had to play the kid-friendly thing for a long time because Westerners see "animation=for children".

But that never happened in Japan. No one was ever taught "this is for kids, and this isn't. Don't mix them", so no one grew up believing that, so they never went on to make it like that, and the cycle of media-styles being for whatever you want it to be continued for them. They have comic books and cartoons with adult characters in adult situations made for adults and written word novels made for children (literacy is also much higher in Japan, so totally written word novels with little to no pictures, but for children, is normal). They don't distinguish the delivery method for a story being based on who the intended audience is for.

Things are changing a bit though, as more westerners have sought out anime because it's one of the last places to get quality story and characters. As Western media in all forms; live action, cartoons, comics, novels, etc all turn to shit because of the woke mind virus, some westerners have been seeking out any form of fictional media that still has good stories with good characters and aren't made as propaganda. And anime has been chugging along this whole time being just that because the Eastern cultures that make it were making hundreds of kinds of it for all ages and groups. They even have sub-names for kinds of anime based on who its made for. There are some anime that are for kids. Shojo and Shounen anime are meant for child girls and child boys, like Western cartoons. But they also have Josei and Seinen anime which is specifically made for adult women and adult men. To them, a Seinen anime show is no different than something like Star Trek DS9 or Breaking Bad, just animated. The fact it's drawn instead of using live actors just isn't a factor for them like it has been for westerners.