Why is it that no nation today still uses the word "Empire" in its official title?

Because the term has largely been de-legitimized, similar to the way "monarchy" has. You don't see many newly created monarchies these days, because the world at large is not comfortable with new monarchies. Despite this fact, new monarchies have come into being, as there are several countries that have been de facto monarchical for several generations (such as Syria, Cuba, North Korea) which have instead come under the term "hereditary dictatorship". However, empires have come into existence as well without naming themselves as such. Unquestionably the Soviet bloc in Eastern Europe was an empire post-WWII, and was for a time (until about the 1970s) organized explicitly as essentially vassal states whose purpose was to benefit the Soviet Union more than anything else, and too the conquest of Afghanistan by the Soviets was fairly clearly an "imperial" move. Nevertheless, communism is ostensibly anti-imperial, so it would not do for them to name themselves an empire, even when it is plainly true.

Other examples might include, say, China, which extends rule over territories which are very much a different society and culture than the main country and which would generally prefer not to be ruled by China (both Tibet and Xinjiang).

Significantly, the concept of "empire" is mostly just out-dated in a modern world. Even in situations where a formerly autonomous region or sovereign country has been conquered and is currently oppressed by another nation the model of empire doesn't entirely apply accurately. Empires are creatures of a different era, one before communications were omnipresent and near-instant, one before travel and trade was ubiquitous and rapid, one when gathering raw resources and taxes from vast lands or selling manufactured goods to a captive market were some of the most certain ways to acquire wealth and power.

Moreover, many of the most largest, most populous, or most resource rich regions of the world today are much more individually powerful than they were during the most recent heyday of empire. A nation such as China or Australia or whom-ever could not merely decide to roll up on, say, Brazil, Saudi Arabia, or even Nigeria or Somalia and have an easy time of conquest. Many of those countries have strong and well equipped modern armed forces with tanks, fighter jets, cruise missiles, and so forth. Conquering them would be enormously costly. And even the weaker countries such as so-called "failed states", like say Sudan or Somalia are still full of heavily armed rogue forces who would make occupation a nightmare. And the benefit had from conquering such places would typically be minuscule. The best places to extract resources from are generally those where there is sufficient local stability and infrastructure to do so in an orderly fashion. Additionally, there's little value to directly controlling the resources of another country if it takes great effort to do so, since those resources will simply be offered on the open market regardless. More tellingly, the economies of the most powerful nations today are so much greater than that of "weak" nations that it would make no sense to expend tremendous military resources to conquer them and capture their economies directly.

In short, the imperial model just isn't very applicable in the modern era. The aspects that made empire enticing for the most powerful nation-states in the recent past no longer exist, war is much more costly and much more fraught with peril and the economic or resource benefit is questionable at best. There are still examples of nations interfering in the sovereignty of other nations, there are still examples of nations effectively ruling other nations, and there have been examples that were more or less clear-cut imperialism without being self-identified as such, but overall the term has just worn out its usefulness in describing power and geopolitics in the 21st century.