My thinking is: The Rift S is 99.99% used for "serious" gaming and PCVR will always be that way. For serious gaming you'll probably need some kind of controllers for the near future, since you want that fidelity and haptic feedback. Now, controllers could be replaced some day by braceletes or what not, but this is not going to happen next year.
The Quest and its successors (i.e. stand-alone VR), on the other hand, is and will be more and more used for lighter gaming, video, browsing, productivity, fitness, social etc. Often times, hand-tracking will be enough without the need for controllers.
Therefore, they focus their resources on bringing hand-tracking only to the Quest.