Updated information, raised from the comment below.
The MAUI ToolBox experience hasn't yet been specifically implemented. What you are seeing appears to be coming from the Xamarin ToolBox and is not designed for this MAUI context. The proper MAUI ToolBox experience is planned for a future release. Keep an eye on the release notes for upcoming Visual Studio previews.
This is expected behavior, as Visual Studio does not contain a Xaml designer for MAUI (nor does Xamarin Forms, since VS 16.9). The Toolbox lists items valid in the currently open designer.
See the .Net MAUI XAML Overview
There is no visual designer for producing XAML in .NET MAUI apps. All XAML must be hand-written, but you can use XAML hot reload to view your UI as you edit it.
There is an open proposal requesting one in the MAUI GitHub repo at Winforms like designer with optional hot reload xaml window #3439