Partially because the Webclipper is incapable of appending. At the beginning, I keep the original notes as well, but to begin a project that incorporates them I typically merge them in an order that constructs the narrative train of thought I'm trying to follow--it gives me a more visual outline of what I'm trying to accomplish. The only time the current merge situation works without finagling is when I'm trying to make a daily log of my activities and I haven't already come back to edit/correct any of the notes I've already accomplished something on.
To give an example of a merging complication I've had; I've been learning leatherwork in my spare time. There are many step-by-step tutorials for creating, say, a briefcase, but I don't have the intention of creating the exact same briefcase as another. In this situation, I have many clips representing various steps, but they're ordered in my notebook by order of discovery of the source material, not order in which the steps are to be performed. I want to make myself a guide to making my own briefcase, so I look at all those clips and I merge them into a singular note to look over and make sure the process makes sense. At that point, I no longer need the individual notes--the project, as far as evernote is concerned, is complete and the smaller notes only complicate any future searches.
There are other workflows out there that I could adapt to, but the workflow I have works fine outside of this software suite not supporting it on this particular OS configuration. If I'm "doing it wrong," why is it not only possible but a feature on a different OS with the same software suite? This isn't a new feature to Windows, mind you, it's been there for at least a year--it's a sign that the Mac app is given much lower priority than Windows as opposed to a sign that it's not how Evernote is intended to be used.