That’s exactly what I think too! I really thought the latest update and expansion pack would finally make this possible, but it was such a disappointment. I wish we could start the game with a single Sim while still giving them a backstory through the family tree — adding pre-existing relatives and established family connections.
That said, the recent updates have made it a bit easier to work around this. I usually create the whole family in CAS — parents, grandparents, siblings, and my main Sim (turning some of them into ghosts depending on the story). Then I start the game, remove the ghost Sims from the household using cheats, and move the other relatives to different lots, leaving only my main Sim as the active playable character.
Still, I really wish CAS offered more detailed family relationship options.