[Cockbrand]

I'd like to add to all of the reasons I find valid (not obscuring what one is pointing at, mimicking pointing with a finger) that everything displayed on the screen is pretty much perpendicular to the x or y axis. The tilted cursor thus sticks out among the rest of the content.

As an aside, as a typical Amiga quirk, the early Amiga mouse cursor was tilted in a 45° angle to the x axis, contrary to all the other popular GUIs with more acute angles for the cursor. And there was a built-in tool for creating custom mouse cursors, which I personally loved. See for example http://toastytech.com/guis/amiga12.html

[Findecanor]

I'd think that the original Amiga arrow pointer could have got its stubby style because hardware sprite could only be in low resolution and 2:1-stepped lines did not look very good.

The style was changed to a sharper point in Amiga OS 2.0 but it was still slanted 45°. First on the Amiga 1200/4000 with AGA could you get a high-resolution mouse pointer.

[teo_zero]

In Workbench 2.0+ it was slanted 45° with the two sides at 30° and 60°, it was red with a hint of 3D (light at the top-right, dark bottom-left) and was beautiful! It stood out on whatever background.

[bhaak]

Including setting the hot spot.

Similar to the Macintosh mouse pointer the click point was not at (0,0) but at (1,1) (the orange point in the skin colored area in the editor on the linked page).