Comic Strips

One could say the same thing about Christianity. Jesus basically ripped his notes from the Stoics.

But the answer is that in both cases there's a lot of supernatural stuff in the texts. When you look at the totality of the scriptures and appended lore Buddha isn't just some wise man who had and epiphany, while early scriptures were closer to that interpretation a lot of the later scriptures that describe enlightenment and elaborate on it... it's basically more describing that people who reach enlightenment get superpowers, mostly omniscience but also like a bunch of other stuff. There's also a lot of writings and different sects that elaborate on the afterlife and how one earns their place there. Like there is legitimately a Buddhist version of hell and it looks fairly familiar to the Christian one because both got cross contaminated with Hinduism's Naraka and depictions of the Greek afterlife just like Christianity did.

A philosophy I think is a discussion about observations of life and how it is lived and particularly opinions on how it is lived well. When you start appending supernatural rewards and punishments to that discussion you get a religion or a cult.