The cells and the packs are different things. The packs aren't just a bunch of cells glued together. See this[1] fun teardown discussion. Another company building battery packs would have different costs even if they were buying the same exact cells from Panasonic, and it's likely Tesla had a hand in designing the cells so I don't know if that's particularly possible.

The reason for the Gigafactory is to compress supply chains, but more importantly because there isn't enough global demand/supply for batteries to support the volumes Tesla wants. Without building their own factory, the global supply would be a constraining factor for Tesla's plans. The Gigafactory is a long-term bet that they will need that much supply in the future. This is not a problem that other carmakers have where they would want to "vertically integrate" oil refineries and gas stations, because availability of gasoline is not a problem.

[1] https://www.youtube.com/watch?v=CpCrkO1x-Qo&t=29m30s