When looking at it after the fact you're actively looking to find the problems and using analytical tools to help you figure it out. While the match is happening few people will be looking to find evidence of cheating based on analysis. Sure, you can have tools actively looking at all times, but that takes extra work, which means extra money.

It's like when you're looking at a bush; only when you're actively trying to count the branches will you actually find how many there are. Only the blatant weird configurations like there being only 1 to 10 branches or so will be that obvious.