The mechanical mouse tends to get dirt in it over time, and track less smoothly. Also, there can be wear on the moving components.

An optical mouse is superior in both of these respects. Until recently, optical mice were very expensive; but in the last several years they've become commonplace.

The bottom of an optical mouse looks like this:

A laser light shines on the desk surface, and its reflection is analyzed for motion.

So the inside is just electronics.

If you move an optical mouse over a completely uniform surface, the electronics can't detect the motion and the cursor on the screen won't move. Early optical mice required you to use a special mouse pad which had a pattern on it which the electronics could detect. Modern optical mice are very good at detecting motion from even the smallest irregularities in the surface, so very few surfaces are problematic for modern optical mice.