Why were Japanese military banners of the Sengoku Jidai mostly abstract shapes, whereas European military banners of the same period (as well as the Middle Ages, 1700's) were so elaborate?

Really? News to me.

European:

Japanese:

As far as I can see both used a lot of plants, animals, religious symbols, and geometric shapes, with a sprinkling of other representative shapes. The main difference seem to be European ones rarely had words as the main symbol on their banners, while kanji characters are common on Japanese ones, which is not surprising due to the difference in the written language. A single kanji can easily represent a family or clan, but a character from the Latin alphabet can not. The European practice of marshalling heraldries to show desendence and inheritance isn't practiced in Japan, but those super complex heraldries don't seem to show up on the battlefield banners anyway.

If anything, European early-modern national flags in battle (stripes and crosses everywhere) were often much less complex than Japanese heraldry, and also compared to Japanese depictions where flags and banners were literally everywhere, European flags and banners were few and far between.