Probably because they seem to be recreating the cursor on the webpage for that cool effect. Even on a good computer I have some input lag, and going from very low input lag and 120fps cursor to that it feels slightly off. Although I might be imagining it just because it looks different than the normal one...
In this case it looks like they didn't just want an image though, they wanted the cursor to invert the color of whatever part of the web page it's over, and to seamlessly morph into a selection highlight whenever you mouse over certain controls. Seems like that's a lot harder to make performant.
You can do that by changing the cursor icon for the elements in question. The CSS rule does support per-element swapping (because of course it does, that’s how a text input has a bar but a button has a pointer).
The only obvious reason I can think of is security concerns (some sort of user confusion), but JS wouldn't help with that. What other reasons are there?
