Just because I was curious, I did a little digging: A cursory look at Sean Barrett's website on the Internet Archive lists Dec. 2006 as the first date where stb_image was published (but looking at the file, versions go back much further than that)

https://web.archive.org/web/20061205030342/http://nothings.o...

That means for 15+ years, he's been iterating on the same file. It certainly works as a development style, but I personally feel that it's easier to edit a set of smaller files.