I hesitate to paint with quite so broad of a brush here since fresh vegetables tossed in oil and vinegar have been a staple of most Mediterranean diets for centuries and salads certainly were not unknown throughout all of Europe starting in the 14th century but to answer your question about your very specific definition of salad largely derives from the Renaissance dish of salmagundi. Originating in Italy but popular throughout western Europe starting in the 16th century, salmagundi was a vast array of seasoned meats and vegetables typically served on a bed of lettuce and dressed with what you would recognize as a vinaigrette. The American concept of the salad takes off from there and from other immigrant culinary styles before seeing a pretty sizable explosion during the home economics boom in the late 19th century in which the salad became an American culinary mainstay.

I am on mobile so I'm going to cut it off there for now but I will add the important fact that oftentimes throughout culinary history you tend to see a spike in salad consumption whenever new fruits and vegetables become available in a culture. The humble salad makes an excellent delivery mechanism for novelties.