When you think about how files are stored on the filesystem it becomes clear why this functionality doesn't exist. Each file is basically a list of blocks and

When you think about how files are stored on the filesystem it becomes clear why this functionality doesn't exist. Each file is basically a list of blocks and some metadata like the total length of the file. What it doesn't have is a length for each block--they are assumed to be full length except for the last, which is stored as the total length of the file. So if you wanted to add bytes to the front of the file you would have to allocate new blocks to store it, but since there is no map of length for each block you would have to only move it by exact block lengths. Same for shrinking the file by cutting off the head, you can't handle values other than full blocks. It's certainly possible to build a filesystem where this would work, but when you wrote programs using the feature they wouldn't be portable to any other commonly used filesystem. People also don't change filesystems very often, so even if you got the change into Ext and waited a decade many people would still be incompatible. Finally, it's a feature that is helpful only rarely. So there isn't enough demand to push through such a massive change given the headwinds it has.