I'm curious, is there a reason why you can't incrementally prepend to files? I haven't seen a filesystem that allows prepending to a file without rewriting the entire file. Is it just because it's not traditional, or is there any particular technical reason why it would be too expensive, or etc?

In a world long accustomed to "append only" files, use cases would be relatively few.

Vs. the folks creating and testing filesystems would have to do a whole lot of work to add that feature. Followed by the folks doing higher-level software, most of which which need to handle files being prepended to.

In short - interesting idea, not worth the effort.

its a legacy from the tape-storage times

the size of the prepended data would rarely be an exact number of blocks such that it became purely a metadata update (see also: inode small file optimisation), and thus you would have a small "hole" in the file between the new data and the existing data, which would have to be bookkept safely.