It is hard because every bit has to be overwritten, maybe more than once.  It takes a very long time for Diskpart to do a "clean all" command because of the size of today's drives. A reset may reformat a drive but it is not workable for most users to be forced to wait hours for their systems to become available. I just did a reset and it gave me the message about not removing all personal files, so if the system is being sold, to run a second reset...
This is for the purpose of returning a laptop. When I reset Windows 10, it says deleting my files will just make it "harder" for people to access them. I understand the files may still be accessible using data recovery software. I'm guessing the reason for this has something to do with the reset process leaving the binary data of the hard drive state mostly intact while only deleting the pointers to that data. If that's the case, shouldn't there be an easy way to just wipe over everything and reset it to zero? I've looked into DBAN. I don't understand what this means, but I hear DBAN works with an earlier version of the hardware BIOS and what I have is UEFI which DBAN doesn't work with. Also, DBAN nukes some things it shouldn't, like the operating system itself, so that needs to be backed up. Shouldn't there be a way to wipe only the data that has resulted from someone actually using the computer?
How do I wipe the hard drive, and why is it so difficult?