
It seems that it's because the Sims who lived there before either sold everything or took it with them when they left.

I usually don't have the patience for furnishing and only put the bare necessities down, and as a result everything looks kind of meh. This is doubly annoying for apartments, since I can't just download a new one from the gallery. Whenever I load up a household to age up a baby (I play with aging off), or I move an active household onto a new lot, the house/apartment is completely empty. That solved the issue for me, hope it does for you, too. If you pack it it ends up in the household inventory. That means the furniture stays at the house (as opposed to disappearing, which is what I thought would happen).

Most houses/apartments in my game have absolutely no furniture anymore. When you move a household, you have to select 'sell furniture'. However, I imagine that even without the mod you would run into this issue eventually, since the only two options seem to be "sell furniture" and "put in household inventory" when you move. I play with MC Command Center and Sims get married and move autonomously.
