to the term "nations" : "A large aggregate of people united by common descent, history, culture, or language, inhabiting a particular country or territory" then there surely is a nation of Arabs given the scores of attributes these congregated lands have in common.
However most countries belonging to the "Arab nations" assemblage has within their borders their own customs, traditions, laws,historical achievements,advancements, etc that may cause them to differ to various degrees from their neighbors. And you may notice many Arabs prefer to be referred to by their own country of origin when it comes to the nationality label rather than simply be known as "Arabs".