The same reason there is no reason to make towers private. One exists for the sole purpose of being a private space; the other exists for the sole purpose of being a public space. Both towers and houses are plenty big enough to where you don't need two of either (there's lots of empty space in my house, and there's still a floor of my tower I've yet to figure out a purpose for).
+1 as I suggested before, instead of having different types of things (house vs tower, SDB vs cabinet), things should be more generic and we should be able to assign permissions instead.
-1 Not only does it defeat the purpose of keys but that's what I presume towers are for, as suggested above. Towers are your public space, houses are your private space. Would you, in real life, just leave your house open to everyone and anyone? Even some friends can't be trusted in your own house particularly the ones that might go poking around your cabinets and the like.
I like it how it is, I have few people that have access to my house, nothings been taken or anything. It works for me, and, I presume, many others.
-1 as mentioned twice before. That's what towers are for. It's so people can have a public place and a place to litter their floor with stuff without being afraid of losing it. I have no idea why people would need 9 floors let alone 12 to give out for selling or even for showing off your style why not just put it in one of your tower floors, but as long as it's optional and you have to choose to use this potion and not have them set you to it automatically and you have to choose not to do it (is that confusing?) then I guess I wouldn't be bothered with it if people want that to happen ( I still don't want that to happen to myself. I have too many things on my floor to allow people to enter my house.)