Tesla designed and makes everything else, and that's a lot. The technology that goes into making sure that the battery lasts a long time, works decently in cold and hot, can be charged at high speeds, doesn't explode etc. is very sophisticated and no other car maker has this knowledge in house.
Even at the battery cell level Tesla doesn't just rely on Panasonic. Browse LinkedIn a bit and you'll see that they have in-house R&D for battery cells, they sponsor well-known cell-level academic researchers and they just spent over $200 million on Maxwell Technologies to get their cell-level IP.
The first thing that stops other car makers from making big moves in the battery space is lack of conviction. They still act like they are only half-convinced that EVs are 100% of their future in very short order (speaking relatively in car business years).
And the second thing that stops them is massive investments needed to build their own battery gigafactories.
Third is probably lack of internal expertise (which Tesla was building since the days of roadster).
Even those most committed (like Volksvagen) project for 2025 maybe 1/4th of (projected) Tesla's production.
Fun fact: Volkswagen wants to partner with SK Technologies on battery factories.
Reportedly their current supplier (LG Chem) threatened to cut them off if they proceed.
It's unprecedented for a supplier to threaten their biggest buyer which should tell you the current distribution of power in battery technology.