Versus Junk Brands
The main reason junk brands are bad buys is because they are short-term buys. After their warranties expire, it's not likely that you'll be able to get them repaired when they break, and very likely that they will not work with whatever new camera you buy ten years from now.
These brands don't make the effort to update years-old lenses to work with new cameras; they step away from the issue like a hot potato and hope you buy a new lens from them instead seven years from now.
When I buy good lenses like Nikon, I know they will last me for decades, always working on my new cameras and repairable if it breaks.
With the junk brands, you seem to get a deal if you only think short term, but when I pay over a thousand dollars for a lens, I expect it to last me for decades.