The word couch appeared in English before sofa did.
At first a couch was a bed, not an elongated, padded chair.
This is because the French word that it came from meant to “lie down.” The Latin ancestor of the French word was collocare and this Latin word in turn was built on two parts that meant “to place together.”
It certainly wasn’t the intention as the word developed, but since more than one person can occupy a couch, having evolved from being placed together, the name is appropriate.
By 1450 a couch was a couch in English but a sofa was still unheard of.
The first citation for sofa came in 1625. It seems that other European countries that had been dealing with Arabic speakers had been using the word for a little longer since in Arabic a sofa wasn’t so much an article of furniture as a raised part of the floor intended to make sitting more convenient.
These built in benches must have been covered in something to make them more comfortable because not only does the first English citation for sofa talk of a rich carpet covering it, the etymology of sofa itself meant “carpet” or “mat,” running back through Arabic and Aramaic.
It was 1717 when sofa first meant “couch” according to the Oxford English Dictionary.