It's not that the glow fades over years, it's that it doesn't keep going without having more light applied to it.

The kind of GITD that is applicable to keyboards is photoluminescence, specifically phosphorescence.

Basically, light hits the substance and makes the electrons happy. They're so happy they throw a rave, and spin their light sticks while rolling on ecstasy.

Which isn't accurate, but is still true in a way.

The electrons get the energy from light, which causes them to emit light over time. The amount of time they can hold that extra energy is limited. In the kind of materials that could be used for keyboards, you'd be dealing with hours at most. There are materials that can emit light for days, but the amount of light after the first few hours is too low to be useful, and isn't really applicable for this.

You can get GITD stickers for keyboards already. They do work, just not really long enough or bright enough to be something reliable in the way a backlight is.

And, the phosphorescence does reduce over very long time frames (way more than years), so having it printed onto the keys directly isn't worth the extra effort to manufacture, since the printing on keyboards would also be removed over time by use as well as the reduction in GITD properties. At least, I don't think it would be worth it. That's opinion, not something based on anything a company has said about why they don't do it.

Fwiw, one of my keyboards has the stickers on it, and they do okay, but only when the board is in decent light intentionally. Indirect lighting isn't enough to get a steady glow that would last for a long writing session. An hour, maybe two, yeah, but not much longer. So, if that's the use case for them that a person has, where the board will be well lit right up to when it'll be used, and won't be used for the equivalent of a full shift of work, they're great. Otherwise, backlights are vastly superior