You could argue that publishing research is not the same thing as doing it. But they don't seem to have done much of it until fairly recently.
I agree that Apple does less research than other big tech companies. But they do it where they think it matters. Their M-series CPUs are more than just integration and polishing. And they have been doing some research in health AI as well, I think.
https://machinelearning.apple.com/research seems to have too many publications to be considered a "dearth" IMO.
But I think part of the problem is that Apple simply hasn't focused on the tasks and the methods and the people that have now turned out to be so impactful.
They have clearly been course correcting for a while now as some of the more recent papers show, and they have done successful research in areas such as health AI.
1) Siri
2) Dearth of published AI research