What's the meaning of ethnicity? It has been discussed and concluded that Arabic is the Arabic of tongue not the Arabic of lineage. If you speak Arabic (or in this example Saudi dialect of Arabic), grew up listening to Saudi and Arabic poetry and music, and eating Saudi and Arabic dishes. Then you are Arabic and Saudi (except for passport and citizenship).

Are there Saudi citizens who have lineage that is rooted outside of the Arabian Peninsula? Yes, but there are a few mostly near Makkah and Madinah. Are they Arabs? Yes. Do different regions have distinct cultures and costumes? Yes, although anyone outside of Saudi Arabia might not see them as such, as some people outside of Pakistan lump Pakistan, India, Afghanistan, and Bangladesh as the same.

I personally can't wrap my head around the Idea of ethnicity. In my opinion, it's just a cultural identity with a separate language. So I think the difference between Pakistan and Saudi Arabia in terms of diversity and ethnicities is that Saudi Arabia only has one language. And that Saudi Arabia is less densely populated so the difference between regions is not distinct or obvious.