Hey there,
Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.
If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (for instance, why did Victoria only become Empress of India rather than "Empress of the British Empire") - this kind of question is more likely to get a response in our experience. Hope this helps!
As mentioned, this kind of question is really hard to give a good answer to, but I'll give it a go.
There are three answers, here:
They were, kinda (Victoria through George VI were Empresses and Emperors of India). As an extra aside, though it doesn't directly cover the title, Henry VIII did declare England to be an Empire: ("... this realm of England is an Empire... governed by one Supreme Head and King having the dignity and royal estate of the imperial Crown of the same" is in the Statue in Restraint of Appeals, broadly as an explicit refutation of the claims of higher (than Royal) authority of the Papacy - this is important, and I'll get back to it at the end). Also, some of the pre-Norman monarchs mixed an "Emperor" in there (Eadwig went for "King by the will of God, Emperor of the Anglo-Saxons and Northumbrians, governor of the pagans, commander of the British": they basically made their own titles up, so they got a bit out of hand sometimes.
We tried, but George III turned the title down in 1801 (discussion of why here, though the actual reason is impossible to tell with any certainty without far more knowledge of his thought processes than we're ever going to have).
(And I think more what you're asking for): neither were anybody else: none of the major European colonial powers called their monarchs Emperors, except when one or another was the Holy Roman Emperor, Bonaparte-era France, earlier France (but only when talking to Muslims, as part of a long-running insult campaign by the Ottomans), Portugal (ludicrously briefly, and only in pretence: they only claimed the title of "Titular Emperor" as part of the treaty recognising the independence of the Empire of Brazil, and lost it five months later when the monarch holding it died), and Germany (if you want to count them as a colonial power, and the title was based on their European holdings, rather than their colonial ones).
The question, then, is why none of these countries decided to call themselves Emperors. There, I think, there are two answers: firstly, none of them had any particular justification for doing so. Titles and the like are heavily based on historical precedent, and none of these monarchs ruled areas that had traditionally been the full territory of an Empire. "I've got lots of extra land a long way away" kinda misses the point, there, and unilaterally promoting yourself above your peers on such a basis would, at best, be seen as spectacularly egotistical (even by the standards of monarchs!). Then, there's the cultural aspect: the title of Emperor in Europe is special. Broadly, the only people in Europe claiming the title of Emperor were also claiming some form of descent from the Roman Empire (however remote: the Byzantine Empire and the Holy Roman Empire being the obvious examples, but also the Ottoman Empire (they claimed the title of "Emperor of the Romans" (which was the only title with the word "Emperor" in it) after taking Constantinople, by "right of conquest"), Austria (claimed (fairly legitimate) descent from the Holy Roman Empire, and hence from Rome), Bulgaria (appointed such by Byzantium, and claiming to be its successor), Russia, and Serbia (rather dodgy, there: they were claiming to be a successor to Byzantium, but Byzantium still existed at the time, and didn't recognise the claim). Further, if you happened to care about the Pope's or Holy Roman Emperor's opinion (which, you know, basically everybody did: at least the latter, given the armies available to most Holy Roman Emperors, both of those were very much of the opinion that (a) only the Pope could grant someone the title of Emperor; and (b) that the Holy Roman Emperor was the only Emperor in Western Europe.