It's because at the time the British monarchy did claim the French throne, based on old claims going back to the time of the Hundred Years' War. English and later British monarchs pressed that claim until the French Revolution, when the French monarchy was overthrown and abolished. They dropped the claim in 1800, after the Act of Union of that year united the crowns of the Kingdom of Great Britain with the Kingdom of Ireland, and formally recognized the French Republic in the treaty of Amiens in 1802. After the Bourbon Restoration, the British monarchs did not again pursue their claims in France, and the French monarchy was eventually abolished for the last time in 1848. I have an older answer which includes the royal styles, which I'll copy and paste below:

The only time a British monarch was also titled Emperor was the period of time between 1876-1948, when the British monarchs were titled as Emperor/Empress of India. The title "emperor" in English usually refers to someone ruling over a multicultural, well, empire: a nation-state that is comprised of people of many nationalities and languages.

Queen Victoria was the first to be styled Empress, and her full style and title at her death was: "Her Majesty Victoria, by the Grace of God, of the United Kingdom of Great Britain and Ireland Queen, Defender of the Faith, Empress of India."

I hate to post stuff from Wiki (bad form from a moderator) but rather than retyping them all, I'll copypasta the titles and styles of monarchs and how they changed over time (EDIT: I should mention that these date from 1707 because of the Act of Union of that year, when Scotland and England united to form the Kingdom of Great Britain.).

Years	Style	Monarch
1707–1714	By the Grace of God, Queen of Great Britain, France and Ireland, Defender of the Faith, etc.	Anne
1714–1801	By the Grace of God, King of Great Britain, France and Ireland, Defender of the Faith, Archtreasurer and Prince-Elector of the Holy Roman Empire, Duke of Brunswick-Luneburg	George I, George II, George III
1801–1814	By the Grace of God, of the United Kingdom of Great Britain and Ireland King, Defender of the Faith, Arch-treasurer and Prince-Elector of the Holy Roman Empire, Duke of Brunswick-Luneburg	George III
1814–1837	By the Grace of God, of the United Kingdom of Great Britain and Ireland King, Defender of the Faith, King of Hanover, Duke of Brunswick	George III, George IV, William IV
1837–1876	By the Grace of God, of the United Kingdom of Great Britain and Ireland Queen, Defender of the Faith	Victoria
1876–1901	By the Grace of God, of the United Kingdom of Great Britain and Ireland Queen, Defender of the Faith, Empress of India	Victoria
1901–1927	By the Grace of God, of the United Kingdom of Great Britain and Ireland and of the British Dominions beyond the Seas King, Defender of the Faith, Emperor of India	Edward VII, George V
1927–1948	By the Grace of God, of Great Britain, Ireland and the British Dominions beyond the Seas King, Defender of the Faith, Emperor of India	George V, Edward VIII, George VI
1948–1952	By the Grace of God, of Great Britain, Ireland and the British Dominions beyond the Seas King, Defender of the Faith	George VI
1952–1953	By the Grace of God, of Great Britain, Ireland and the British Dominions beyond the Seas Queen, Defender of the Faith	Elizabeth II (before coronation)
From 1953	By the Grace of God, of the United Kingdom of Great Britain and Northern Ireland and of Her other Realms and Territories Queen, Head of the Commonwealth, Defender of the Faith	Elizabeth II

So you can see that the first monarch to take the title of " of the United Kingdom of Great Britain and Ireland and of the British Dominions beyond the Seas King" was Edward VII. This reflects the "Dominion status" of Canada and Australia, and was carried forward until Elizabeth II's coronation, when her title/style changed to reflect the Commonwealth nations.

Edit the Second, by the grace of God the last: the reason for the title Empress of India was because of the political reorganization in India after the rebellion of 1857, when the old British East India holdings were incorporated into Britain. The title was awarded to Victoria by an act of Parliament in 1876, and the title Emperor/Empress was held by British monarchs until Indian independence, in 1948.