Most people have no idea how to detect basic non-blatant cheats. Even simple low-FoV aimbots sneak past the vast majority of players. Good enough cheats are virtually undetectable by just human inspection.

The few people you saw cheating were probably frustrated enough with the game they decided burn their accounts and ticked enough boxes in their cheat menus to let them go out with a bang. It may of course be that you are particularly good at spotting cheats and Halo Infinite is just blessed to be relatively cheater-free. (I've not played it.) But I'd expect 1-2 orders of magnitude more games to have cheaters than your estimate.