Why won't bootable DVD image files burned on USB stock boot?

Question

Suppose I have DVD iso file. As far as I understand, these non-hybrid iso files don't contain GPT or MBR ar the beginning, instead they have their own speciffic file system and some partition table. Is that true?

What difference does it make if I burn this image byte-by-byte with dd to DVD or USB? Why doesn't firmware (BIOS/UEFI) beheave the same way when it tries to find bootloader on the device, be it HDD, SSD, USB or DVD? If it can boot from DVD, why wouldn't it boot from other devices the same way if it contains the same image?

Edit: Note that I talk speciffically about DVD image files. Hybrid image files work properly, since they contain GPT or MBR at the beginning and look just like HDD layout.

Answer 1

Because it's an ISO file.

While ISO files are simply just a DVD "image", it's technically a single file until it gets burned to a DVD. Once you burn the DVD, it is no longer an ISO file.

During the process of checking your bootable drives, the BIOS is looking for a directory that meets certain criteria. Your ISO file may contain an image of a directory, but not an actual directory.

So how do you circumvent this?

Simply by extracting the directory and files from the disc image onto your flash drive. Your BIOS/UEFI is extremely limited on its capabilities; as in that it doesn't have the software to provide it the ability to read image files.