Many games are held online. In person, computers get smaller all the time (and there are bathroom breaks etc.). The history of trying to cheat at chess, casino games, and more with concealed computers is far too large a topic for one reply.
That didn't prevent Anna Rudolf being accused that she hid a supercomputer in her lip balm box made of tin sheet metal in 2007 and cheated with it in the bathroom.
Some parts of the chess community are very ridiculous.
This was unsubstantiated and took a similar toll on Niemann. The recent Netflix documentary interviews the key players, and it’s pretty that he didn’t cheat in that situation. Magnus’s cozy relationship with chess.com made Danny Rench jump to conclusions, giving the Magnus team the impression that cheating had happened, but a later chess.com analysis showed that was unlikely. It made for a good viral story, but wasn’t fair to Hans, no matter how unpleasant his personality is.