Byzantium not being in the base game feels weird only because it hasn't been done before. That was the same with the Ottomans, the same with the Iroquois (both in the base game in Civ V), Sumeria (in the base game in Civ VI), etc etc. Similarly, Civ VI saw many fan favorite civs not returning. We shouldn't always use the past as a predictor, as the likelihood of something happening doesn't necessarily increase over time. Sometimes, expecting a key result (i.e. that swans found in Australia will be white, like all the swans Europeans saw in Europe) doesn't help correctly predict anything (the swans were black). Maybe we are used to Byzantium not being in the base game, but that doesn't mean Civ VII won't surprise us. Civ VI certainly surprised us by including Australia in the game at all, let alone Canada, and by excluding Maya, Babylon, Portugal, etc.