I'm 100% sure that if Facebook could have an iOS app store, they would start one. It's worth all the work for the improved tracking alone, as app download ads are a big part of Facebook's mobile revenue.
From a technical perspective, I'm not sure how hosting littler apps within your own app, which is already on the Apple App Store and subject to its review process and rules, would allow you access to improved tracking.
On the subject of a Meta third-party app store independent of Apple's control- yes, there is motivation there to do that, but I am dubious of how much of a threat that is to the end user because:
1. Apple still maintains control of iOS and can restrict invasive tracking from the OS level.
https://news.ycombinator.com/item?id=30199125
2. I'm not actually convinced that Meta, Google, Amazon, et al are really capable of executing successful alternative app stores. They would need to make it a sufficiently seamless and friction-free experience, and offer enough incentives for users to overcome having to sign up for yet another service just to use the apps they already have access to.