Different modes of co-ownership.
In the building you’re talking about, each company has/rents some distinctive piece of real estate.
Not the case with gigafactory.
E.g. read there: http://fortune.com/2015/11/03/tesla-grid-batteries-gigafacto... As you see, Tesla’s saying they will be producing cells, not Panasonic.
I mean, serious question. In the office building I work in, my company only owns two floors. The next floor is another company, and the floor above that is yet another company. My company owns more than half the building (the bottom floors has more space for various reasons), but the stuff made upstairs isn't made by my company.