It's the spindles themselves. PS5/PS4 tends to be the worst, because the spindle is so tall and flexible that any compression on the center of the case will cause the disc to pop out inside.
You can test it yourself: open up a PS game case and simply press down on the spindle. Game pops right out!
The standard Xbox cases are better in this regard because the spindle usually requires you to push up on the disc at the same time to remove it. (Some older Blu-Rays use a slightly flatter version of this spindle.)
Some other Blu-Rays have a different spindle type, but like the Xbox cases, the lower height means less travel — like pressing a button — so less chances of compression in shipping, which would mean less free discs.
