How can it be that IBM was able to make long lasting keycaps decades ago and modern manufacturers can't?
Oh, that one is easy to answer.
How is it that keyboards used to have individual mechanical keyswitches long ago (the Dell AT101W, the Apple Extended Keyboard) and now they can only make rubber dome keyboards?
It's not a question of "can't". If a keyboard isn't wireless or otherwise fancy (i.e. Optimus Maximus, or even just Luxeed), it's a low-end $19.99 item, made as cheaply as possible in China. So if you put a wired keyboard on the shelf for $50, they would laugh at you.
But wait. You're talking about keys getting worn on your Filco? That might make it a harder question. Particularly as ABS is what keycaps were made of in the grand old days of cupped keys with double-shot molding, so it's not an inferior cheap plastic. But those keys were made smooth, not textured.
POM
is superior to ABS, but it's an exotic plastic. Failure to use POM instead of ABS can be put down to a keyboard maker seeking merely very good keycaps, and going for superlative only for tactile feel.