It's because the developers have extremely fast and graphics accelerated hardware. I can't notice any lag whatsoever on the mouse cursor on my M2 Pro but.. But a lot of folks won't have top end HW.
m1 pro using firefox, I can notice the cursor not being smooth, and if I move the mouse while scrolling then it lags hard
I have no cursor lag and I'm on a 2019 Intel MBP.