Check out the letter "A" in the image. Why is it smudging / wearing off?
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.
I've had this exact same issue, and it is always those 3 or 4 keys. It happened to me just 6 months after owning the Macbook Pro this year (2020). They replaced the board, but by Oct it started again.
I don't accept that it is based on how you type tho. I had a Macbook Air for about 7 years or so, and sold it looking almost like new. It is how they are made. Whether they are made cheaper, I don't know.
Anyway, it happened again and Amazon came to the result as I got it from them. I've now got a replacement unit. Different keyboard too, so hopefully it's better quality.
For Apple you expect it to be the best. They always say they do the best, not the cheapest.
But yes, I've had that issue, TWICE within 12 months.
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.