Standard DocType cannot have default print format, use Customize Form

I understand the above message for standar DocTypes. But why is it the case for custom DocTypes in custom apps?

The standard is always the predefined auto-generated print format (which actually is not a print format at all). If you want to customize the print format, you will need an actual print format record. You can then set this to be your default in the customization (the “Standard” will remain tough, don’t be bothered with it)…