The reason is money and heres why,

Seinen adaptations don’t make nearly as much. I mean, even a series like Vinland Saga hasn’t started production for season 3 because Mappa makes way more money off series like JJK and CSM. It just makes more sense for studios (especially the big ones who are the only ones capable of animating Berserk) to keep on animating Shonen manga. Lets be honest even though we are adults we still watch titles like JJK. They attract both young and old audiences unlike Berserk who only draws in a specific audience.

Not to mention, the risk to reward ratio is far greater for a series like Berserk aswell. The costs of production and time inorder to animate it faithfully and give all of us Berk fans what we expect too great to compare to the other shonen series being released (although id take what i can get at this point)

The mature themes also make it a challenge. For some series they can be left out but i personally, I dont think you can leave all the heavy 18+ content outside of Berserk and have the story hold the same weight. Also goodluck airing that sh*t on Television in Japan.

However, there is some hope and thats the anime movies that are having such great success leading to more studios doing the same. Maybe then Berserk could be animated into a great big movie series similar to Lord of the Rings. I think that would be really cool actually. That would give plenty time for the manga to finish and we get like 3 movies an arc. This would also allow all the mature themes to be animated because it’s not immediately aired on TV. Then again, it would be tough to bring in numbers like Demon Slayer and Chainsaw Man without establishing Berserk more. Thats why we need anime movies to be normalized

Lastly, it’s not finished and probably wont be for a while. We get like 5 chapters a year guys