The historical reason
The accepted answer traces it to the earliest mouse work, where the pointer was an upward arrow, and to the machine built at Xerox PARC where it acquired its lean. The constraint was pixels: a straight vertical edge and a diagonal at forty-five degrees are both easy to draw crisply on a coarse grid, whereas an arrow symmetrical about a vertical axis needs shapes the grid cannot render cleanly at that size.
Two further reasons it stayed
- Cheaper clicks. One answer noted the tilt put the pointer's tip at the top-left corner of its bitmap, which is where a bitmap's origin already sits — so working out where a click landed needed no extra arithmetic. Small, but it ran on every click, on hardware where that mattered.
- It stands out. Another answer made the case from vision: the early stages of visual processing pick up orientation among other basic features, and a shape leaning at an angle separates itself from a screen composed almost entirely of horizontals and verticals.
The practical side effect
The person asking had already noticed the useful consequence: the tilt leaves the pointer with a perfectly vertical left edge, which is exactly what you want when placing a cursor between two characters of text. Whether that was intended or inherited, it is the reason the shape still works for the thing people do with it most.
The one that is guesswork
- One answer suggested the shape reads as a hand pointing at the screen, and said plainly it had no evidence beyond its own impression.
- It is included here because the thread included it, and because the difference between the documented reasons and this one is worth seeing.
- The resolution explanation is the one with a source behind it.