Rare? See below.
Tell that to: Alexander the Great, Roman Empire, Egypt, Mongol Empire, Persian Empire, Han Dynasty, Ottoman Empire, Umayyad Caliphate, Spanish Empire, British Empire, Russian Empire, Holy Roman Empire, even the United States. Many lasted for centuries and all conquered huge parts of their known worlds.
Alexander frequently put the former local ruler back in power as a vassal or tributary, rather than imposing outside rule. Rome took several centuries to steadily advance and assimilate the regions it occupied militarily, despite an advanced road system and strong emphasis on logistics, as well as setting up trading posts and forts in the north to gradually accustom the barbarians to Roman civilization, and still had to abandon much of that eventually. Egypt mostly saw itself as either one or two kingdoms throughout most of its history, so it wasn't so much a case of "outsiders" as civil conflicts. Persia and Assyria both made massive expansions, and faced constant internal struggles for centuries, both of them ultimately collapsing.....and so on. Look at where the HRE is today, how well the vast British Empire has held together worldwide, and how the Ottomans have maintained control over the culturally different Balkans: they ultimately failed. Most of the large empires of the past survived as long as they did, and grew as quickly as they did, by leaving most of their conquered territories at least semi-autonomous, as long as they paid tribute and furnished levies on demand. Full annexation in the modern sense wasn't practical, or in many cases, even possible.
The common themes are that either the people already saw themselves as part of that larger culture (China, Germany), were gradually assimilated after a long period of looser control and trade, were taken in many small bites over several centuries, or had no strong central authority to begin with (British in India). None of that holds true in most of Europe, other than for German unification, and to a lesser degree, Italian unification, in which there was a long sense of common cultural identity. In the few instances where a country rapidly annexed huge swaths of a territory which perceived itself as being a part of something else, that country either collapsed or was eventually forced to relinquish the annexed land.
Victoria 3 should allow one to take more in a peace settlement than was possible in V2, BUT the consequent militancy at home, infamy abroad, need for constant suppression forces, and the financial disincentives of paying to maintain land and occupation forces with little return should make it pointless to do so except where the specific circumstances make the steep costs worthwhile. V2 had WAY too many revolts in core territory, and not nearly enough in non-core territory that wasn't actively being militarily suppressed. Pacification of annexed non-core territory should REQUIRE a military garrison of some kind, or face almost certain revolts, leaving the conqueror with a lot less troops free to make further conquests. Vassalization, a puppet government, or incorporation into one's "sphere" should be the norm during the timeframe, rather than annexation, except in cases such as retaking cores. Colonial expansion into more primitive areas should fall into "colonization", rather than "conquest", with land grabs from "partially civilized" countries falling somewhere in between.