This is mainly human brains being notoriously bad with randomness. They think random, but expect something else entirelly. Given the many rolls over a playthrough, let alone across multiple ones for hundreds of thousands of players, even hugely unlikely events become a dead certainty to happen to someone.
https://youtu.be/MEewLWDpscA?t=1500Aditionally, confirmation bias. If your brain is bend on that something fishy is going on, it will focus on the many times the dice went against you, whilst the times the opponents had bad rolls are being ignored. There are many way more elegant (and less player trust breaking) ways to make a game harder, some of which quite easy to implement, plus having a rigged dice is not D&D which the game is trying to emulate.
