John R. answered 02/25/20
Classics PhD; Expert in Greek and Latin; Broadly experienced tutor
Think of it this way: they are all Latin. The Roman Empire controlled such a vast territory and 'Romanized' so many different peoples that Latin spoken at one end of the empire (more accurately, the western half of the empire), say, Italy, began to sound different than Latin at the other end of the Western Empire, say, Portugal or Spain. Eventually, these peoples, each influenced by local dialect shifts and even contact with other languages, could not really understand one another; so it made little sense to talk about the language they were speaking as the same language. This is how the romance languages came to be, albeit a simplified account.