Limiting piracy is the ongoing reason, but there is also the historical reason of the Video game crash of 1983 which led to Nintendo's Seal of Quality.
Essentially as the platform owner, you want to ensure games sold for the platform "just work", and if you have a bunch of third parties running bad software, consumers would lose faith in the platform altogether.
Essentially as the platform owner, you want to ensure games sold for the platform "just work", and if you have a bunch of third parties running bad software, consumers would lose faith in the platform altogether.