Your question about the sense to play the tribes and cultural groups as cohesive nations, is a good question, and I could answer to that : Cultural groups and tribes are unplayable in the game.
But Your two exemple are really wrong !
Visigoths and Ostrogoths are definitly Kingdoms and not "tribes and cultural groups".
You Rise a good point : Where is the limits of a tribe and a kingdom, a nation and a culture. Well could I make Sarmats or Venedes as playable ?
The other point is : some state are not recognized by the Roman Empire, or the in other time by a great empire, But does that mean that they did not exist? As now when The USA recognized or not the Palestinian state, China, or North Korea, this countries exist after all.
At all events, Even if the limits between tribe and kingdom are not clear, Visigoths and Ostrogoths are kingdoms.
If you talk about culture groups, talk about "goths".
When they are kingdoms, dynasties and capitals, coins, armys and laws, official religion and politics systems, I can't think only one seconds they are tribes and not able to play.
Well if you want to help, try to add some new informations, events, propositions, graphism, etc. and not soustract.