Almost all of my hobby project programs are in a single file. With right use of comments and organisation of data structures and functions it is not much different than dividing the code into multiple files. I often have multiple views on the file open, just like you would with multiple files.
meh - at some point when LLMs are groking giant contexts without issues, we can just make programs with a backend, frontend and data layer all in one file. There's no reason not to.