You may be mistaking cabbage/lettuce for another key ingredient in kimchi that is very foreign - chilies! Chilies are a new world crop that didn't exist in Europe or Asia until the Columbian exchange in the 15th century. read more
The Japanese had limited contact with Europe at this point and were able to introduce spicy chilies to Korea. This means, of course, that prior to the end of the 16th century, kimchi was completely devoid of any heat (beyond that coming from ginger and garlic). read more
Note: I'm not Korean, but I know a lot about Koreans. User-10416669202261656891 has a good point when she pokes fun at the question through comparing kim chi to french fries. It's a strong parallel, but I believe french fries to Americans is nowhere as beloved as kim chi is to Koreans (from Korea anyway). read more