You'd have to ask Apple. Probably because they just want to sell more watches
I'd guess 2 things:
1) From the less-than-stellar quality of Whoop's app, their data is probably differently formatted and they don't want to bother
2) They want you to stay in their app and use their algos to manage your health data as opposed to Apple's, which they can't control.
Also, to counter one of your arguments above, Apple Health doesn't do a lot of analysis on third party data short of showing it in charts, so there is still value to using Whoops app for that aspect of it.
As an example, my scale is Whithings, my bed had a Beautyrest Sleeptracker both of these services consume and contribute to Apple Health.