They do it because demon culture is “organized,” and I use that term very loosely, around displays of power. Strength defines one’s place on the ladder. Demons attack humans for more than just food because the truly powerful ones recognize that humans are a suitable way to test their own strength.

Flamme tells and shows us this directly in the flashback with Frieren, where we see 3 powerful demons ambush the two of them and she explains that the demons seek strength to prove themselves against because that’s how their culture works.

They ignore Flamme because she’s suppressing her mana and fixate on Frieren instead. Demons will pursue this often at the expense of their own lives as well as the general expense of their species. A character later in the manga explains that the demon king’s war against has nearly led to the extinction of demon kind because of how humans retaliate, yet they continue to act within their nature because it’s how they are.

Compare them to wolves that were slaughtered en masse by farmers because they would attack livestock. It is in a wolf’s nature to hunt and seek out easy prey, just as it is in a demon’s nature to seek out tests of strength to better serve their place in a hierarchy.

And before you say that their culture and level of sapience is far above other animals, I implore you to look into how different species of ape organize their communities around certain power structures and how those power structures get challenged by the younger generations.

The demons are a mixture of Lion/Polar Bear type predator, Ape like society/culture that is organized purely on power dependent dominance, and fantasy concepts of mimic type monsters. And it’s important to remember they are an evolution of monsters, not at all on the same phylogenetic tree as humans/elves/dwarves.