Comment by metroholografix

Official chess competitions are taking place under stringent monitoring conditions and even then, with professional reputations on the line, there have been multiple high profile cheating incidents.

Amateur online chess on the other hand is besieged by cheaters that use engines, even in casual non-ranked games where there's absolutely nothing to gain besides a pat on the ego. This has drastically changed how the game is played today with lot of players gravitating towards speed chess (bullet and blitz) to compensate. That will thin the herd of cheaters but one still runs into engines on a weekly basis.

This is also the tip of the iceberg, with the true scale of the problem being orders of magnitude worse, as someone dedicated enough can use an engine to cheat in a way that's essentially undetectable.