Problem is, you could pirate every single game on dreamcast. Just get a legit copy of the game (renting, buying and returning, borrow from a friend), and have a CD burner.
Then you could make a 1:1 copy of the game in roughly an hour.
You make it sound trivial. While Sega left a security hole open for games to be loaded from a regular CD, the official games were released on GD-ROMs, a dual-layer CD with a 1.2 GB capacity.
So first off, you couldn't read them completely in a regular CD-ROM or even DVD-ROM drive. (I'm not counting the "swap" method because it's failure-prone and involves partially dismantling the drive and fiddling with it during operation.) You had to connect your console to a computer and use some custom software to read the GD-ROM on the console, and send the data over.
Once you had the data, you then had the problem of trying to fit a potentially 1.2 GB GD-ROM image onto a regular CD-ROM. A handful of games were actually small enough to fit already, and 80-minute and 99-minute CD-Rs would work in the DC and could store larger games. But for many games, crackers had to modify the game files to make them fit.
Often they would just strip all the music first, because that was an easy way to save a decent amount of space. Then if that wasn't enough, they would start stripping video files, and/or re-encoding audio and textures at lower fidelity.
Burning a CD-R from a downloaded file was easy, but ripping the original discs and converting them to a burnable image generally was not.