Eric Hobsbawm basically lumped all three under the heading of the "short 20th century" (1914-1991) and characterized it as the Age of Extremes — an age of illiberal ideology, technology, globalization, mass-destruction and slaughter.

Which is of course one way to parse it. As I point out every time something like this comes up, historians can make periodizations of many different sorts. None of this kind of categorization should be mistaken as "natural" or necessary — it is what it gets you, as an analyst/historian, that matters. Lumping World War I in with WWII and the Cold War does get you something, it is an argument about continuity, about technological development, about the failure of international systems to prevent war, about ideological-based conflict, and so on. But it also loses something: World War I was not World War II (even if it carried with it, in its resolution in particular, the seeds of the second), World War II was not the Cold War (even if etc. etc.), and so on. Just because things have continuities and repetitions of themes do not make them identical. World War I, World War II, and the Cold War all need to be understood on their own terms, and even those categories are tricky (the Cold War is especially so — when did it start? when did it end? Did it end? Was there one Cold War or two? etc., but even World War I and II have trickiness in defining their boundaries) as well. But you can get things out of implying they are all part of an extended period of conflict, especially (as one might imagine a future historian to make) in a rough generalization of the 20th century (in the same way one might roughly generalize the 16th century or the 11th century, doing some damage to the fine-grainedness of the past).

So the question is not, "why would historians lump them together" — that's maybe something one might ask a student on an exam, to see if they can spot thematic continuities (and also one would hope they could point out the problems with such a schema as well). The question is: "what do you get out of lumping them together? And what do you lose?"