When was the last time you actually used. NET? Because that's absolutely not how it is. The. NET runtime is shipped by default with Windows and updated via WU. Let alone that you're talking about .NET Framework which has been outdated for years.
The issue is in supporting older windows versions - which sadly is still a reality for most large-scale app developers.
.NET runtime is not shipped with Windows, but once installed can be updated by WU.
Only the latest .NET Framework 4.8 is shipped with Windows at this point.