My question is :
Why Microsoft still use Windows form to build desktop app ,Instead of creating new frameworks that makes it always up , they just keep old frameworks alive
Windows Forms is one of several Microsoft technologies with which you can make desktop applications (see https://dotnet.microsoft.com/en-us/apps/desktop). Some of those technologies/frameworks are quite old like Windows Forms but others are pretty new (like .NET MAUI see https://docs.microsoft.com/en-us/dotnet/maui/what-is-maui).
One of the reasons Microsoft still supports old frameworks like Windows Forms and WPF (and even migrated them to .NET Core) is that many desktop applications were made in those technologies and are still being used. Many of them are still getting new features. This was a surprise even to Microsoft. Not supporting Windows Forms any longer would upset many companies.