The default is for a form (as per its title), not individual lines on a form. And certainly not for lines that don’t exist yet. It is a tool to save work in filling out a form, not a place to set default content for when new elements are added to a form. The default form can be virtually as complex as you want, but it isn’t going to make decisions for you on what to add when additional content is inserted.
And for every user who might, like you, appreciate the tax code from your single default line item being duplicated, there will be someone (or many someones) who finds that totally aggravating. It could easily lead to mistaken entries when a user neglects to change an entry that, at first glance, looks to be complete.
You want it to add a default tax code, which might seem obvious to you. Someone else might have a standard non-inventory item set for all sales invoices, such as a diagnosis fee for an auto mechanic. Would that person want every additional line added to a sales invoice to be for the same diagnosis fee? Probably not. But that is what would happen if content from one line on a default form were duplicated when new lines were added.