At its core, Salesforce is a platform. As such, our customers expect their code to work for the long run (and backwards compatibility forever). Not owning the framework fundamentally means jeopardizing our business and our customers, since we can't control our future.
We believe the best way to future-proof our platform is to align with standards and help push the web platform forward, hence our sugar and take on top of Web Components.
Also to your comment about using different frameworks, again as a platform, allowing our customers to trivially include their framework choice of the day, will mean that we might end up having to load seven versions of react, five of Vue, 2 Embers .... You get the idea :) Outside the platform we love all the other frameworks (hence other properties might choose what it fits their use cases) and we had a lot of good discussions with framework owners about how to keep improving things over the last two years.
Our goal is to keep contributing to the standards and push all the things to be implemented natively on the platform so we all get faster and better.
More coming soon :)