Perhaps they did it to save RAM since you are buffering the current sector anyway. An alternative is CP/M's extent-based filesystem, where the meta-data is all in the directory entry. But I think space allocation algorithm is more complex for it.

I like the OS/8 file allocation scheme: files are always contiguous. Only one growing file can be open for writing and it allocates space out of the contiguous free space (actually it owns all of the free space until the file is closed, where the unused space is then given back). If you run out of disk space, you have to compact the disk, eliminating space taken up by deleted files.

I like it because it's illustrates the surprising fact that you can get away with only one file open for writing for most things.