Good question! Almost 80% of EV batteries are manufactured in China, while about 10% are manufactured in Europe and 6% in the US.
The main reason for this is the demand coupled with the access to the raw materials. A vast majority of rare earth metals–a key component of EV batteries–are mined in China which makes it easier and significantly cheaper to transport ready-to-use materials to battery factories.
Not only does China have some resources on the land, but the country also does a lot of refining for battery materials imported from other regions. Once refined and ready to use, battery manufactories are easily able to access materials to create batteries for the EV companies internally.