What factors make World War I and World War II distinct from, say, the Seven Years' War? Didn't that conflict also involve great powers and span multiple continents? Why don't we call that a world war?
To start off, Winston Churchill did consider the Seven Years' War the first world war.
I would argue that World War I and World War II are different from earlier conflicts due to the fact that both wars involved all the major powers FROM AROUND THE WORLD. Conversely, the Seven Years' War and other wars like it only involved the major European powers and their colonies.
For a world war, it is important to distinguish between world in a political sense and world in a geographical sense. Regarding the Seven Years' War, it was a world war in a geographical sense. There was fighting in Europe, the Americas, and even Asia and on the high seas. However, both WWI and WWII could be considered world wars in the political as well as the geographical sense mainly since those wars basically included all of Africa as well as major Asian powers such as the Chinese, the Japanese, and smaller states (both independent and colonial possessions).
To be honest, this is more of a theoretical justification rather than a technical or historical one, but that could be a key difference between what we know as world wars and those "world wars" that are not called such specifically.
hi! you may be interested in this earlier post
Why didn't/don't historians refer to the Seven Year War as the first world war?
... and this one, which is more of an addendum, summarizing the involvement countries from the Americas in WWII