(Several characters in the novels comment on this, so it is entirely cannon.)
The Culture fought a war against the Idirans, who eschew the use of intelligent machines. The Idirans lost.
Similarly, the Gzilt civilisation in the novel The Hydrogen Sonata prefer to run their ships with a quorum of uploaded humanoid mind states running at highly accelerated speeds.
Humanoids in the Culture are controlled at every level, even their thoughts as guided… sorry… “aligned” with the Minds through the adoption of the language Marain.
No humanoid ever overrules a ship Mind in the novels.
They're an anarchistic society, no one has power to overrule any one else.
https://www.reddit.com/r/TheCulture/comments/kbtylt/ysk_the_...
Just as shippers can successfully take two characters who clearly have no basis for a romantic or sexual relationship and pair them off in their head canon, likewise people can disregard the author's statements about their work and decide it's completely contrary to how they've decided to understand it.
Some people look at https://en.wikipedia.org/wiki/Fountain_(Duchamp) and they do not see a sculpture.
It doesn't.
>struggle [...] hardwired into living beings, without which they wither away
The reason Sisyphus is happy (or so we imagine) is not because he's struggling for all eternity
Both Vyr Cossont and Ngaroe QiRia are so bored with their meaningless existence that they have to go out of their way to invent “life tasks” and other challenges to immerse themselves in.
Not to mention the scene where a ship mind casually crushes the spirits of Vyr by perfectly playing a piece on its first attempt that she struggled with for years!
While there are some types of people that would be bored and would struggle in Culture-style utopia, I think for overwhelming majority of people it would still be a massive improvement over today's world.
Nope, they have considerable freedom, arguably greater than we have today. It's true that they cannot directly challenge the almost onmipotent Minds when it comes to stuff that matters to them, but Minds in Culture are an example of a very well-aligned AI, so it's almost always the case that going against a Mind's intent will just make the situation worse for everyone.