TypeScript is also there thing. They've been pushing React as well in SharePoint & other areas for quite a few years.

Blazor is very new & was considered experimental until recently. It's still fairly bleeding edge.

Microsoft started Teams in 2016 & bought Xamarin in 2016.

JavaScript developers are also more plentiful than Xamarin I assume.

They should choose the best tools for the job, even if it's not there own.

- Edit - It looks like WebView2 is a Microsoft thing based on their Chromium browser version. Super common for Microsoft to make multiple competing products that do similar things.