Why are grad classes generally 3 units?

I feel as if they are much harder than undergrad classes. I have only taken 1 grad class in CS and 1 in math so far, but I have felt like grad classes are as hard as ~2 undergrad classes combined.

This is not meant to be complaining or anything like that, just genuine curiosity about why they are counted lower. I know that many graduate level classes are pretty light with grading, but there are also many grad classes that are just like undergrad classes (final exam + regular homework). So why are they counted less?