HDD don't care about opening files. If the FS supported the same operations, I could send grep directly to the block level and have it scan contiguous lists of blocks that cover files I care about. You will get false positives for the overscan (potentially), but you will also achieve the raw rate of the disk (150MB/s for spinning and 500MB/s for solid sate).

So in effect, these big data serialization systems are attempting to lift the file system into userland so they can make just that optimization.