This has been addressed several times... and even though I agree with you on the fact that we have too few furniture slots (and houseguests / pets / mounts are counted as furniture lol) to decorate our houses properly, the limit exists for performance issues.
It sucks but that's the reason. Or at least, that's the reason they give.
More so as its something they sell.To me it seems strange to sell this feature, but not have room for it.