Mobile website redirect to desktop site?

If visiting a page such as https://en.wikipedia.org/wiki/Main_Page on a mobile device redirects you to https://en.m.wikipedia.org/wiki/Main_Page then why doesn't the reverse ever happen? I.e. sending desktop users away from m. subdomains.

I understand that the basic answer is "it requires more programming", albeit minuscule, so why is the effort never put in?

The m. subdomain was the de facto method for presenting a mobile site to visitors and I don't recall ever coming across a site that would direct desktop users away from m. subdomains even though they often times looked terrible, barren, or downright broken on a desktop.

My question stems from lack of experience in ever developing m. websites so I am hoping for some insight from others that had to develop m. sites.