Check out the letter "A" in the image. Why is it smudging / wearing off?
The keys on Apple's laptops are translucent, with black paint applied. The letter is 'reversed out' of the black paint. That's so the backlight can shine through the letters. So it's not white paint smudging, but the black paint wearing out.
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.
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.
Seen this on many keyboards of many manufacturers - it’s about how you “hit” the keys...
With a slight sliding action you will eventually wear away the material.
You could use a keyboard cover to protect the keys from further wear, but some say they don’t like the different feel - that’s a choice to make.
And, you are not alone - been happening on mine as well... As in the shift key and the command key.