It isn’t that they can never get adapted well, it’s just much harder to do. I think Fate/Stay Night has examples of terrible and great adaptations. Studio Deen’s adaptation of the Fate route was awful, their adaptation of Unlimited Blade Works was even worse. Ufotable’s adaptation of Unlimited Blade Works was pretty good, and their adaptation of Heaven’s Feel was great.
Mushoku Tensei was adapted amazingly, and while many details were left out, the adaptation is a masterclass of how to do it well.
If we’re talking about ordinary books, Andy Weir’s The Martian had a pretty good adaptation, and I am expecting Project Hail Mary to be good as well.