That doesn't seem obvious to me. It's plant milk, as opposed dairy milk or animal milk. Some plant milks have been used and known as 'milk' in the English language for centuries.I suppose that anything that is made using almonds, soya, oats is obviously not milk.
1390 – “Almond milk” is first mentioned in English in The Forme of Cury. It came to be widely used in Europe during Lent – the first popular non-dairy milk in the Western world.
From this page: History of Soymilk and Other Non-Dairy Milks (1226-2013) - SoyInfo Center which gives its source as the book 'History of Soymilk and Other Non-Dairy Milks (1226-2013)' by William Shurtleff, Akiko Aoyagi (ISBN: 978-1-928914-58-7), Publication Date: 2013 Aug. 29.1704 – Soymilk is first mentioned in English by Domingo Fernandez Navarrete in his book A Collection of Voyages and Travels. Navarrete served as a Dominican missionary in China.