It's an interesting question. Some thoughts:

• Generally, cognition is aimed at solving problems, so we are drawn to think of downside risks and conflicts.
• As a second order effect, conflict and downsides are more memetically fit. I can easily name media imagining bad results of reprogenetics (Gattaca, Brave New World, Sandel, Habermas, Fukuyama, etc. etc.); harder to name media imagining good results.
• As Kaarel notes, genuine utopias are also alien, and our real values largely route through developmental processes (learning more, understanding more, reflecting more, empathizing more, regularizing through childish understanding, etc.).
• As a corollary of the previous point, contra the Anna Karenina principle, there's something that's much more relatable about pain, suffering, failure, problems, death, conflict, and bad outcomes generally. There's many ways to fail, but they all have stuff in common. In contrast, consider free creation of a life in general. Or as a metonymy, consider the free creation of a piece of music. There are so many degrees of freedom and an infinite range of structures to express in the music. Cf. Fun theory https://www.lesswrong.com/w/fun-theory.
• Hope is painful. Thus, the infinite hopes of children are worn away with age; and for an adult, regaining hope would be painful.
• Cf. "Border Guards" by Greg Egan. https://www.gregegan.net/BORDER/Complete/Border.html

It is like people are unable to picture utopia the same way someone born in the 1900s could probably not picture pocket computers or anything on the internet in full detail.

That's easily explainable because pocket computers are dystopian :-)

Seriously though, I think the general problem is that we are machines for doing things. Imagine a utopia for cars: a universe filled with car washes, where cars are cleaned with soft brushes all day. A few roads too, optimized for being fun for cars. Does that sound like a good use of the universe? Hmm. But what would be a better use of the universe, from a car's point of view? Hmmmmmmm.

It'd be easy to change the problem, assume that we're machines for getting enjoyment. Then utopia would be a universe filled with enjoyment. But we aren't such machines.

It is like people are unable to picture utopia the same way someone born in the 1900s could probably not picture pocket computers or anything on the internet in full detail.

an important reason very very very good worlds are hard to picture (especially in full detail) is that they are very far away from us in development time. like, i think there would probably be more technological/economic/social development between now and very very very good worlds than between the big bang and now. these worlds would be extremely hard for us to make sense of (though ultimately not meaningless). also, my guess is that these worlds will still be developing; this would thwart attempts to conceive of them as given/finished things

or you might be asking why people find it hard to picture any world that is even merely much better than ours, and not necessarily very very very good or near-perfect. in that case, my comment is less of a response

Plenty of people and collectives have written detailed depictions of utopia or related, like the Elysian's dozens (hundreds?) of essays and Max Harms' 70+ posts. Also Richard Ngo's characterising utopia, Cleo Nardo's stratified utopia, and plex's utopiography although these are more conceptual.

The disagreement you mention is a separate issue. Holden looked at reactions to various efforts to describe utopia and concluded "You can emphasize the abstract idea of choice, but then your utopia will feel very non-evocative and hard to picture. Or you can try to be more specific, concrete and visualizable. But then the vision risks feeling dull, homogeneous and alien", then followed up with a framework for visualising utopia that avoids these problems by describing a spectrum of utopias from "the status quo plus a contained, specific set of changes" to the sort of radical utopia you find in trans/posthumanist fiction.

I don't understand why the probable likelihood that many / most people won't like your story genuinely portraying your version of utopia hinders you from trying. The set of concrete visualisable descriptions of utopia everyone today likes is empty.

I agree with Kaarel about envisioning something clearly that is very distant from what is current.

But I also wonder if there is not another issue here. Whose utopia are we talking about. While I doubt I could write out a complete utopian world for me I am pretty certain that many would take exception to it being a utopia because they see things differently. So unless one is talking about some private world of their own (or small number that agree with the structures) making a utopian setting is a big public choice problem. We cannot even solve getting good policies in many cases, reaching for utopian outcomes seems a stretch.

There's a strong evolutionary bias for rejecting things that are too good to be true. Many times throughout history, technology has appeared to promise abundance, but this hasn't come to pass, and many of the people preaching imminent utopia have had ill intent. Automation was a core facet of USSR propaganda, for instance.

On a less emotional/heuristic level, finite matter in the universe is a more imminent constraint than most people realize. I did the math a while back, and if all of humanity had the birthrate of Eritrea^[1], we'd end up with more bodies than the universe has atoms in fewer generations than humanity has already experienced. Barring the ability to create more matter, we cannot promise abundance and freedom to everyone indefinitely. Depending on one's politics and philosophy, this means that utopia is either bad because of the tacit implication of some kind of eugenics policy, or bad because it amounts to strip-mining the universe to avoid one.

1. ^{^}

   Genetic and cultural factors that increase birthrate are both heritable, and even low-birthrate societies have high-birthrate subgroups. If anything, this is a conservative estimate for what will happen in the long term when resource constraints on reproduction disappear.