Furthering the other replies, manufacturing is
hard.
In a similar vein to the way software devs roll their eyes when a client says: "..It's just a tiny change, it won't take any time", there are so many hidden gotchyas when you're working with physical products.
If you're randomly picking a factory off Alibaba and thinking you've got it nailed, you're very, very wrong.
Furthermore, when you're making physical products, there's no undo button. Some errors cannot be corrected. Didn't plan your mold perfectly?
Oops, the 10k USD and one month that you sunk into your mold .. gone.
Don't have someone on the ground that actually cares about you and your product? You're going to get screwed, even 'unintentionally' - the factory probably won't care that this batch of plastic is grey, not black, but your customers will..
Over the years, I've worked on delivering dozens of physical products projects in China. The biggest rule to learn is ego: when to apply it, and when not to apply it. (And typically, you should very rarely apply it..)