Why is maple syrup less viscous than other liquid sugars?
Maple syrup is more liquid because it contains more water.

That also depends on the type (category) of syrup; most consumer syrup will be on the lighter side of it and will contain more water as the darker ones because it will be less concentrated.

You could dillute honey or Golden syrup with water to have a lighter syrup but the cookies will taste different.

Maple syrup grading in Québec
http://fpaq.ca/en/federation/quality/grading-system/