Minor (historical) detail: In fact, some operating systems *do* use an EOL marker in files. For example, in CP/M, the directory entries tell you which sectors hold data, but not how many bytes are in your files. So text files would use a ^Z to mark the end of the document. That carried over into DOS, so in DOS programs you'll often find some ^Z characters in files.

Thank goodness we no longer have to deal with *that* mess!