Because I am using one as a christmas tree decoration and people look straight into it every day.
Unless the mouse is labelled as a class 2 laser device, then it will not cause damage to your eyes. Most (possibly all) mice are either not lasers or class 1 laser devices, which are safe under all conditions, and do not require labelling.
If you want to know a lot more detail, there is a company called Broadcom Inc. (formerly Avago) which owns lots of patents in the this area. They have produced a detailed document covering aspects of safety with regard to optical and laser mice [PDF].
Well, I did spend some time experimenting with this few years ago.
I noticed that after spending several minutes staring directly at mouse LED at close range (as in 1-2 cm), the way I perceive colors changes. It resets back to normal after few minutes of not staring into the mouse, and as far as I can see, there was no permanent damage.
I don't think the LED is powerful enough anyway. You may get blurry eye for a few moments but you can get that just by looking at a bright light bulb for a duration. Your retina can get a little bit tired if staring in it for too long but this is all from the strong light. And it is not infra-red. Humans cannot see infra-red light. It is visible red which is higher in frequency.
Point source emissions such as lasers and LEDs are generally not good for your eyes.
And if LEDs are bad for your eyes, I'm not sure what makes a class 1 laser less harmful.
Best to avoid looking directly at it.