This behaviour is expected because the custom inspector builds its own rectangle and draws a button on screen without going through the normal IMGUI layout system.
As a result the IMGUIContainer that is auto-created for IMGUI inspector has no idea about the size of the IMGUI content, so it ends up having a height of 0, therefore not receiving clicks.
The fact that the hover state is responding is misleading, this is because of how the hover effect is implemented (it is not based on mouse events, it just changes accordingly if the mouse is over the object in the repaint code).
It should be possible to get the desired effect by using GUILayout.Button.