If you add a "System: Wait 3 seconds" action in event 2 (before the "go to layout" action) in the Preloader event sheet, you actually do see the progress bar at 100 %, and 3 seconds after, there is the move to the next layout. (seen in preview mode when the "Preloader" layout is active and you execute preview from it)
So apparently it seems there isn't enough data at the time to have a "proper" loading time.
Make sure to also make "Preloader" the "Default layout" in your project properties.
At the moment it is set to default, and due to the order of the layouts, "Main" is executed as first/default layout of the project.