Microsoft promises things included with Windows will be supported ~forever. Adding modern .NET into the mix would break this promise and add more churn.

Automatically uninstalling .NET runtimes would break apps, and Microsoft will be to blame, not app vendors who failed to upgrade to the latest .NET. An app built for .NET 8 can run on .NET 10, assuming no backwards-incompatible changes in the runtime and system libraries, but this behavior is opt-in.