It probably depends on what you expect from an anime. Frieren isn't your typical shonen action where characters are screaming and fighting each other with absurd powers - it's more of a slow burn that focuses on characters and nostalgia, the sense of time passing, relationships that last. In 27 years of work I've learned that people appreciate things when they're well-built, and this anime is exactly like that. The basic plot seems simple but the way they develop it is sophisticated, with an emotional depth that a lot of fantasy anime don't have. The dialogue is smart and doesn't try to be super cool in a forced way.
And then you have to say that the manga it's based on is really well-written, and the adaptation was done with care. Most fans talk about it because it touches on themes that appeal to older people too, not just kids - melancholy, the journey, searching for meaning. Cat lovers like me also appreciate how the characters are characterized with small details that matter. If it doesn't click for you right away it's not like you're missing something, it just depends on everyone's taste. Maybe it's not your anime, and that's fine.