I think you underestimate the influence of both ancient Greek philosophers and Christianity in the formation of modern Western philosophy. They didn't usually just made things up, and even in the cases when they did, they were trying to find support for it in the works of philosophers and theologians of the past.

Even with art it's not as simple as depicted. Until the early 20th century, every school of art (at least in painting and music) was built upon (and drew inspiration from) a previous one.