Grass build up

[Mtsoxfan]

I have a Cub Cadet zero turn, but I'm sure many mowers have the same issue. As seen in pictures, there are openings in the deck around spindles that allow cut grass into pully area. No exaggeration, I just filled a 5 gallon bucket, with compressed grass. It's a pain in the butt using a blower or manually taking off guards. Is there a reason for these holes? My only guess is to allow air escape for high lift blades. If they were plugged, wouldn't the grass keep getting mulched?
Is there a reason I shouldn't find a way to plug holes? Has anyone tried this?

 

[Runner]

What model is it? I have a CC ZTR (Ultima ZTXS 5 54), but don't have holes like that on mine.

Have you always owned the machine? I wonder if the spindles have been replaced and there are parts missing, or maybe wrong spindles?

Mine gets a TON of grass built up on top of the deck, but the belt guards are open on the sides, so it's pretty easy to blow it off with the air gun.