there is absolutely no technical problem doing it on unmounted filesystem but it just imposes certain restrictions that I dont think are worth their cost consi

there is absolutely no technical problem doing it on unmounted filesystem but it just imposes certain restrictions that I dont think are worth their cost considering in many cases you want to copy live files, you have to copy them to place you can unmount, so now you have 2 problems if you dont gzip the dd you risk pure cosmic microwave background bitflips and no checksums, and they can corrupt superblocks and etc if you gzip it (to have crc sums) you cant restore partial files from it so easily unless you use squashfs and significantly complicate the process so tar and gzip is more robust (compared to the dd) just because of the crc (of gzip not of tar), size and portability