From what I understand, it probably has more to do with the way Chinese manufacturing works.
I know a lot of complex items, like small engines and chainsaws, are not manufactured all under one roof. Many parts may be outsourced to specialised factories then assembled into a complete item elsewhere.
Same with electronics, one company manufactures capacitors, another resistors etc.
When you want to build something that uses these items, you don’t tool up to make every single component. It’s just not efficient to do so. You may spec certain items to a certain standard, or just go generic and cheap.
Same with cast items that require foundries and machined parts that can be made from standard tube sizes. Then there is heat treating and finishing applications like anodising. Parts that require special treatment like reflectors and glass lenses.
You start with a design you like, then look for ways to make it work under a certain cost. The higher the sale price, the higher quality you can build into manufacture.
Below a certain level, then close enough is good enough. As long as the item works. When follow up orders are made, profit is taken into account. Perhaps you can save a few cents on each item by making some part/s differently, or the factory you source them from makes them cheaper and is different from the previous one.
Chinese production centres are like little cities with dozens or even hundreds of manufacturers. It’s almost a miracle that they make anything complex that works as well as it does.
They don’t just have a factory producing flashlights. A machine shop makes machine parts. One day it’s flashlight tubes, the next it may be small engine parts.
Orders are processed as they come in. Every new production run requires re-tooling. One size is the same as another to a machine. Say perhaps, that you find a source of 25mm alloy tube stock that is 2/3 of the price but a slightly different size to your last run. You have a renewed order for fifty thousand flashlights. What would you do? The thought that previous customers might want to swap parts with the new model does not come into it. Unless of course that is a design parameter.
The way most people see flashlights as throw away items reinforces the concept. Bin the old one, buy a new one. The new one is probably going to be better in some way, brighter or more efficient. Shiny and new. Smaller or lighter. And cheap enough not to matter.
In those circumstances, who cares if the threads are different. You got a good deal on some tube stock and that improves your profit margin. Maybe you got a good deal on some machine tools for thread cutting that aren’t quite standard and only work with the matching cutting tools. You buy them too, because it improves the bottom line. Price. Who cares if the threads you make don’t fit anything else but the lights in this weeks production run? Cheap and cheerful
I have seen $200 dollar garage jacks thrown away because of a leaky $5 seal. They use non standard seals and you can’t buy a replacement here to fit. Thats not even getting into the different thread standards. There are quite a few as it is.
Ultimately, it’s a result of cost cutting during manufacture. Sometimes it’s different standards. Sometimes it’s just lack of care. But without it, we would not have these lovely cheap lights.