This is exactly the thing. Spiral staircases involve new design and animation taking place across a vertical rise in space. They are difficult. In Sims 4, that animation also needs to include different walkstyles/emotional walks, because we have more variety in how Sims move than Sims 3 did, so that adds to the design and animation work. The vertical rise part can involve new technical development, so that's another group of people to involve. All of these people have to be doing this in a coordinated way -- their timing needs to be right. Add in the design and making of multiple styles of circular staircases. Now add in the tech and design so that those stairs make sense at different wall heights and, to be really well done for this game, multiple stories. Add in some things like children sliding down banisters, Ghosts gliding down the staircase, Vampires flying in a circular path, and you have yet more complexity... but you'd feel the absence of those details if they weren't included.
And then, because new build mode elements have mostly been free (see the previous stairs addition, for example), that all has to fit outside a pack and have funds/budget of all kinds allocated to it. Just because most of those things came together at some point in Sims 3's 5 years doesn't mean that all of those things (because of the considerable added design and animation) have come together yet for Sims 4.
Probably the easiest way to budget a large, complex-to-organize addition like that is to make it part of a pack development. (I've worked on projects in my, less technically complex, creative industry that used project budget to help finance new development.) That adds its own complications, because part of what's being developed is being given for free (unless it isn't), and because it then also has to coordinate with all of the other parts of that pack and be on time. If some things don't work out well and the circular stairs have to be delayed, now you have a big pack element that needs filling in with something else, and you have to redesign anything in the pack that used those stairs.
Now add in that this is just one of many such complex requests to implement, and they can't all happen at once. So what do you manage to fit into a pack in a way that feels like it's on-theme first. These stairs? Bunk beds? "Freed" babies? Those are equally complex. I'm sure, from what SimGuruLyndsay said, that these things will happen... when they can. But it's just not that easy. You can't just plop them in whenever you feel like it and expect them to not be quarter-baked unsatisfactory token attempts.
