Karl - I think you've answered your own question. You don't HAVE to add water, but I think you'd find that you would need to squeeze a lot of plums to yield a gallon or more of fermentable juice. And after you'd done it, I think you'd find it too strongly-flavored (and possibly too acidic). Of all the things that can be fermented, only grapes really have the right balance of sugar, acidity and flavor intensity to make wine purely from the juice. If you make wine from anything else, you are bound to be adding sugar, adding (or reducing) acid, etc. The amount of fruit per gallon is to some extent a matter of personal taste - the published recipes can vary a good deal, but reflect what reasonably experienced winemakers have found to produce pleasant and drinkable results. If you are curious about what your wine would taste like with more plums and less water, feel free to experiment.
Doug