That doesn't stop Apple needing control, or at least feeling that way.
> Apple could as well contribute whatever improvements it deems necessary to benefit themselves and everyone else
They do. Apple uses OSM data and contributes to OSM indirectly (https://wiki.openstreetmap.org/wiki/Apple).
> It would've been a much better starting point, too
Who's to say? As stated, Apple does use OSM data. They use data from multiple different sources. (http://gspa21.ls.apple.com/html/attribution.html)
> than whatever proprietary data they licensed to get started
I detect a bit of a fallacy in what you're saying, that Apple wanting to have control over their own geographical data must mean that they aren't using or contributing to OSM. It isn't a zero-sum situation.
Because Apple learnt a long time ago that something essential needs to be in their hands; third parties let Apple down.
Google let Apple down by refusing to allow Apple to craft vector-based maps, only allowing Apple to download bitmaps. Thus Apple Maps.
Intel let Apple down by failing to predict where the market was moving since about 2012 or so, with the Core processors not gaining much in terms of performance and somehow managing to guzzle more energy. Thus Apple Silicon.
Plenty of other examples where Apple realised they can't offer the whole package until they own all the components. The formula is "<Company> didn't do <Thing> and it impacted Apple's offerings. Thus <Apple Thing>".