They didn't use to. For the longest time cores and memory amount were unknown on iOS devices. Now that they use Mac chips and can tier them easily, they've started fragmenting the iPad Pro line with more storage buying a better processor. I suspect the reason they do this on the Air is so people can directly compare this to the older iPad Pro model or see it as the same as the Mac. It wouldn't be surprising that down the road you'll be able to upgrade out of the binned chips with higher storage even on the iPad Air models eventually.A good point
Why do they even mention how many cores in marketing?