It's a "random" guess. But in my experience, for a website, if it doesn't look good at first sight, a user might just leave instantly. As someone pointed out in the comments, just looking at sites like:
I'm sure that's part of it, but the primary usability studies I'm aware of have all focused on speed/response. Whether this translates to "native" apps or not, I'm not sure, but certainly for the web, response times are king [1]. Curiously, this metric has been a stable predictor of retention for decades (see the sources in the link as it'd be redundant for me to post them here).
To be annoying, and not to disagree with your comment in any real way at all, I’d argue it’s the engineers who are being irrational for choosing to ignore or not make the effort to understand human nature. I’m sure there are plenty of studies in psychology, social psychology, or anthropology explaining the rational basis behind why fashion/marketing has such a strong impact on human behavior. I’d also bet that the advertising industry is well aware of these studies ;).
https://riverbankcomputing.com/software/pyqt/intro https://electronjs.org/
Your eye will probably be more attracted by the good design of the electron website.
I'm not saying an app should be only good looking, but it's clearly a major part.