Wikipedia was one of the worst offenders, but lots of sites screwed this up in exactly the same way, and I feel it was a predecessor to modern "mobile first" web platforms that either treat desktop as second-class users or actively don't want desktop users.
Like the subdomain was RIGHT THERE.
I sort of remember some of the older MediaWiki desktop themes looking worse than the mobile theme, but it was never enough for me personally to try always using the mobile site at the time. I do still strongly prefer old.reddit.com... For as long as that portal continues to exist.
IMO this isn't a good reason. Developers can change the user agent.
(I also imagine there could be a no-redirect preference for logged in users. Or even just a special query string you could add to the end of a url.)
Your website might want to present a different interface for people using mouse and keyboard than for people using tiny touch screens? Even if the number of pixels in the browser window is otherwise the same.
That is not at all the reason; did you read the article?.
Also web developers can just use devtools to simulate a mobile browser.