This is one of those correlation != causation things, hm?
It might be more a case of the “average” Arch user being more sensitive to small quirks/bugs or certain defaults. Arch is at least comparatively unbiased, which might be why these users pick Arch in the first place.
I would personally agree with where Arch is because I prefer a distribution that mostly works out of the box and already made a lot of the decisions for me that I don’t want to be bothered with. I do still customize quite a few elements to my (sometimes very specific) liking, but I also like that I don’t have to do anything when it comes to configuring my disk layout, or configuring zram, or install and configure fwupd or other packages that kind of just make sense to have.
But I don’t really see why Arch users can’t be as happy with their choice as I am with mine, unless the only reason they “use Arch btw” is that they think that’s unironically something to brag about (or peer pressure, but that shouldn’t be a thing I hope).
This is just fun with statistics. I don’t think your Linux distro has a big impact on your overall happiness in life, but of course you can order the results by any parameter you like.
Often, it’s a third factor that influences both, in this case probably age, which influences happiness and distro choice.
Even though most people don’t agree with the stats, I think it makes sense because Arch users are never satisfied with their setup. It could cause many of them to choose an average number.
I’m done recommending stuff, because my use case is not necessarily your use case. I can only tell you that Brave is the sweet spot for me, at the moment.
You’re assuming that Arch causes the unhappiness. Maybe unhappy people naturally tend to use Arch, so as to avoid further pain from painful distros like Pop! OS?.