The shorthand is that because the Prussian leadership realized that while they had France under wraps everyone else was, well, not so much. If they took too much they threatened the 'balance of power' that they knew Britain was very, very, very keen on taking. Bismark spent a lot of time and effort holding off the Russians, Austrians, and British (especially the Russians and Austrians) in just the unification of the German states; asking for even more would be seen as war mongering and only bring more war that she was not prepared to wage.

Further the people she was primarily trying to bring into the fold, Bavaria and Wurttemberg along with the rest of Southern Germany, did not want to be part of the greater German Empire very much so a lightning victory against a common enemy turning into a dragged out war of attrition against old friends is all that would be needed to turn them back into Austrian arms or Russian support. To put it even shorter the Prussian leadership realized they just won it all after going all in; going for more would have meant going all in instantly in the next hand and hoping for the best. They were smart gamblers and got out while they were ahead.

This question has a deeper meaning though in that is missing the point of the Franco-Prussian War and the First World War. They were fight over different reasons. It's like asking why did America fight Britain again in 1812; if they had won in the Revolutionary War why go again and not ask for what they wanted back then? Because it was fought over different things. The first was fought over independence and the latter was fought over impressment of sailors.

Similarly the Franco-Prussian War was fought to brow beat Bavaria, Wurttemburg, and the rest of South Germany into the fold along with beating down the only Great Power (France) who couldn't be sweet talked/threatened politically into allowing German unification. The First World War was fought in the context of Germany already being a Great Power and that already changes everything. It was less Germany goading another power into war for personal, unification reasons and more 20 years of power struggle and power blocs coming head to head; hence the analogy of the proverbial powder keg.

Austria-Hungary wanted to use the assassination of Franz Ferdinand to justify a war of aggression on Serbia and Russia wasn't letting the last standing Slavic state fall under A-H feeling she was their 'protector'. Germany wasn't going to let A-H fall under the weight of the obviously superior Russian army so she mobilized in tandem to protect her ally and believing France would go to war in accordance with Russia (seeing a chance at 'revenge') would also rapidly attack France. You see attacking France wasn't the primary goal of the war (although they did have some aspirations in the West) it was just meant as a side-show for the main showdown at Russia. It just turns out they weren't so lucky twice. So yeah, while Germany had some aspirations in France it wasn't the primary focus of the war; protecting German interests (read: beating down the Franco-Russian-British power bloc and expanding her colonial empire) was.