When we say a berakhah on the lighting of a candle, it is different from other berakhot. It is one blessing in which we do the deed first and say the berakhah afterwards. (We do this especially on Shabbat and holidays because the blessing is said to be what starts Shabbat or the holiday, not the lighting of the candle, and we can’t do the lighting once the holy day has begun.)
The berakhah itself says “… lehadlik ner shel Shabbat” or “… ner shel Yom Tov” or “… ner shel Hanukkah” despite the fact that we may be lighting more than one “ner.” And the authorities have had mixed things to say about this. In some families not only two candles are lit on Shabbat, which are said to represent two spouses, but one additional candle is lit for each child. Some sources say we light one “to honor Shabbat” and one for domestic tranquility. Rabbi Steindel mentioned (as he passed my office and I imposed upon him) that Lekha Dodi tells us to “observe and remember,” two distinct tasks marked by two candles, he mentioned the practicality of having one lit should the other go out, and he said they are two equal candles, thus one light. Other sources have made other allusions to dual purposes.
According to Wikipedia, some say that Jewish women lit Shabbat lamps without a blessing until the 11th century, when someone may have introduced a blessing over the Hanukkah lights. Others, though, say the blessing was the same as one found in the Siddur of Rav Amram (who was the head of the Jewish Talmud Academy of Sura in the ninth century). Later Rashi’s granddaughter, Hannah, described her mother lighting candles and reciting a berakhah. (Rashi lived from roughly 1040 to 1105.)
Some offer the opinion that “ner shel Shabbat” should be translated as “Sabbath light” because light does not come in numbers, just quantities. (Your correspondent is not convinced. I’ve always been a proponent of photon theory.)
And there are some who opine that we simply cannot change the berakhah to suit the current practice.
The answer to this question may have to remain simply “because we do.” Enjoy the candles, and the day.
And at the end of the day we can ponder why Havdalah brings the berakhah “… who creates the illuminations [plural] of the fire.”