Night City is very big and you will be able to enter many buildings, but not all of them. We're putting quality over quantity and like Cargan said - we don't want to have empty buildings in the game. Most of the places you can visit serve some purpose to the story we're telling.
For both immersion and exploration I hope we will see some places that we can explore that is not story related. For example an apartment complex where some places can be entered but not all. With some interesting loot hidden away at some places. It is important (IMO) to add a variety of explorable spaces that adds a feeling that this is an actual city with people living in it.
The problem that arises if all interior spaces are 100% related to a quest at some point, is that we don't have any places that are only there for world building.
I understand your sentiment. That from a player and developer standpoint, adding a shitload of buildings without purpose just to add them - will be too time consuming (for devs) and feel pointless for players. However. I do think it is important to add some places scattered around which can be explored for loot or just to immerse with the world. Because if you don't have any explorable spaces outside of quests at all - I think you would risk players feeling that the city is too lifeless even with a hundred quests in there.
The poison lies with the dosage. If you feel you have to "fill the gameworld" with places like this, I agree, it would be counterproductive. But none - and you could risk players feeling that exploration is not rewarding in itself. I think, if the dev team has the time, that it would be a good idea to add some (but not too many) explorable places with some hidden loot or such. If not for the loot, then for the worldbuilding and immersion. I personally think it would be worse - not better - to add none outside of quests. Because from a meta-perspective that means if a place can be explored there is a quest there at some point.