My 2 cents
Coins grading is based in several condition factors and often times coins are wayyy older than most other collectibles. As from human touch can damage the coin I understand the casing to protect such.
Comics/Figures, again are items that can be damaged, usually older, and much more fragile. Your interaction lies in playing with them unlike coins where they can just be observed.
Video games are fairly new yet and are interactive and unlike other things that are collected they must be plugged into something else in order for them to work and contain much more stimulation (debatable) versus other collecting methods.
Also, its 1 thing to find a sealed like flintstones for NES and grade it but a whole nother to take a game that would be 1$ loose and pay 25$ to grade it.
Just my opinion