Why do I need to do this?

How come I have to do this:

    {{var:year is={current_date format='Y'} }}
    {{ theme:partial src="default" folder="blog/{var:year}" taxonomy="false" since=null until=null }}

Instead of:

    {{ theme:partial src="default" folder="blog/{current_date format='Y'}" taxonomy="false" since=null until=null }}

??