@WeissenSocken88 @BowsacNoodle @LurkPerry @noyoushutthefuckupdad It was due to the MIL-CD exploit. Basically, Sega created a CD format that allows interactive content to music CDs when placed in a Dreamcast. There weren't that many MIL-CDs, but hackers realized that you can just use the game's ISO as the MIL-CD software and play pirated and homebrew games.
But it's not enough. If you look into the Dreamcast homebrew scene it's mostly 2D games or hacks of existing 3D Dreamcast games. Apparently, there is still not enough documentation on the system.