I think, and I could be wrong, that they want each Sims game to be its own. Like, they want it to be unique. I like the idea but at the same time, I do miss a lot that we had in Sims 3. One thing was swatch sharing. You could take the pattern of a couch and put that as tour shirt if you wanted. Or the material of a bed and add that to the police car you just stole. So much fun and interesting ideas with Sims 3 swatches.
It could also be a marketing strategy. If Sims 4 was a continuation of Sims 3, people wouldn't have a reason to buy Sims 3.
So yeah, there are many ways to speculate but at the end of the day, it's all we can do, which really gets us nowhere.