Because plenty of people were arrested for entering the Capitol on January 6th before Trump left office.
You'll just get blank stares. About 12-15 people were arrested prior to January 20. President Trump had PLENTY of time to do a blanket pardon. Instead, crickets.
Rightly or wrongly, the Presidential pardon power is essentially "bulletproof". When I posted circa summer 2022 the number of people arrested prior to January 20, the excuses made for President Trump not doing so were fast and furious.