Why does the lettering wear off MacBook keyboards?
The keys on Apple's laptops are translucent plastic, with black paint applied on the surface. The letter is 'reversed out' of the black paint, backlit through the translucent portion.  So it's not white paint smudging, but black paint wearing off.
Those keys are pressed millions of times. And each press involve some degree of friction, which means the removal of matter. The sweat from your fingers is slightly acidic; and who knows what other oils and particles might get transferred from your sandwich. It's possible - or even likely - that a laptop keyboard might get other 'matter' spilt or splashed onto it.
Any or all of these things might act as a combination of solvent and abrasive. If you use a microscope or macro lens you will see this chipping clearly.
Obviously, the designers know this, and try to make a keyboard whose paint will resist the rigours of a lifetime's use. However, nothing is impervious. How old is your Mac?
As to why this key and no others? A is certainly a key that's going to get more finger-rest and usage than others. It's just the perfect storm of the right amount of wear with a particular mix of particles and chemicals.