...But only if you want a straight line, target-sight-eye. Yet the phaser is almost invariably operated at waist or chest height, not eye height. Why should the line between the three points need to be straight?
I mean, the sight is a prism-like "vision block" anyway. For all we know, it's set up to show the crosshairs at a convenient angle. Or, if it's an active display rather than a passive prism or mirror, it may show the crosshairs over the target at any
angle, plus additional aiming aids and other relevant data.