One thing I've never understood: why don't, say, jam manufacturers use standard-size mason jars so that they can be easily re-used when empty? They can just as easily attach branded labels to standard jars, right?

Or even just standard mason-jar size threads and diameters so at least the lids and bands are interchangeable. Washing and re-using jars is so much easier when the lid doesn't have to be paired with the exact jar it came with. (I know that proper mason jars use tempered glass, but I'm just talking about containers to hold leftovers in the fridge or dry goods on the shelf.)

My best guess is that they're worried about liability and so intentionally make the threads incompatible so people can't use them for canning, but I have no evidence this is actually true.