You know, Tesla (let's ignore everyone's opinion about Musk, although I'll always question why Grimes ever got involved with him, really, I thought she had better taste) had to partner with Panasonic and between them sank ~$5 Billion into building Gigafactory 1 and 2 (pretty sure I remember it was the first but I'll give them the benefit of a doubt) to make 18650s for their car battery assemblies.
Now two points. First, a company the size of Tesla needed an established manufacturer like Panasonic as a partner to know how to build them, and two, those factories were big, serious money.
You might be able to throw together a fly by night little battery plant in Shenzhen without that sort of money, but if you want to manufacture high quality high current batteries it isn't easy or cheap.
Edit: I'm sorry. I told myself I wouldn't post it but I'll never sleep if I don't. I so wanted to have that last sentence read "manufacture high quality high current batteries by the car load ". There, it's out of my system.