For software-eating-the-world reasons, I'd guess. A chance to differentiate your product as software does more and more, versus a big chunk of the experience being within someone else's control and the underlying vehicle being an interchangeable commodity.
I haven't tried it personally, but people seem pretty passionate about the software on the leading electric vehicle brand, plus its associated do-everything phone app letting you control the vehicle, find a charger, or even order a new model.
The situation will probably be like smart TVs for quite a while: lots of janky experiences and user-hostile behaviour, because it's just so valuable to own a software platform that every company tries.
