Why did people believe that water was an element? Some cultures, like in Hinduism, still believe there are five elements: Earth, Water, Fire, Air and Ether.
It really depends on definition. We assume the word “element” has a specific definition that is universal in the English language, when that is not the nature of English. English evolves differently in the different areas of the world where it is spoken. Thus the letter we pronounce “zee” (Z) in America is pronounced “zed” everywhere else in the world that they speak English. Aluminum in the States is Aluminium in England. A gallon of milk in America is not the same size as an Imperial Gallon of milk in the U.K. In India if you say, “I am liking you too much,” it is a compliment. In the United States it would seem like an insult.
In the Hindustani version of English, an element is a state of matter. Thus you have solid, liquid, fire, gas and the mythical ether they say space is made of.
So I believe the answer to your question is it’s a question of regional definitions of the word.