I really want to know why this is feasible for the Quest but not the Rift S, given that both have Inside-Out tracking (ie. IR cameras mounted on the headset). In fact, the Rift S has 5 cameras to the Quests's 4 so it should be at least as capable shouldn't it?
Internal company politics and prioritization of Quest. It's possible that hand tracking will come to Rift S, but it might be a side effect of wanting to support it with the Quest link cable.
I own both and demoing my Rift on a high spec system my friends were blown away and the first question was "Can it run on my macbook?" Then thats where the idea of owning VR died for them.
Demoed my Quest and one person bought one within 2 minutes of taking the headset off, took his to a family BBQ and two more people there bought it after trying it for a few minutes.
I still enjoy many PCVR games but this is just reality.