The reason why it's called condensed milk is because to make the condensed milk, the water in the milk is removed or condensed from the milk through evaporation.
The removal of the water from the milk through evaporation is what makes the milk thick and concentrated.
Then sugar is added, creating the very sweet and syrup like product in the desserts that the condensed milk is used in.
The resulting very sweet and syrup like product known as condensed milk is what gives it the full name of sweetened condensed milk.
Although it's mostly just called condensed milk.
And the high sugar content and lack of water in the condensed milk, create a long lasting, stable and dense dairy product.
The process of turning milk into condensed milk includes, evaporation, adding sugar and canning of the condensed milk.
To make condensed milk, fresh milk is heated in a vacuum evaporator, which removes around 60% of the milks water content without scorching it.
Then a large amount of sugar or around 40% to 50% is mixed in, which creates a thick and sweet syrup.
The mixture is then canned and sealed, which gives it a long shelf life without refrigeration needed.
The condensed milk name directly reflects the process in which the milk has been condensed or reduced in volume and in it's most common form, sweetened.