Because of course they're just on the flat top, where cats like to climb and stand.

I've tried the upside-down rolling chair mats and even DIYed a barbed-wire mesh from chickenwire; nothing deters them. It's a soothing place for them to sleep atop when running.

Condensation forms under textbooks and other flat planks, which will eventually short the controls as water pools on top.

The only other thing that "worked" was having the exhaust from a 1U rack server pointed at it. The heat initially attracted them but when those fans kicked on, cat met ceiling.