Because many languages today are basically offshoots of ancient languages.
Imagine if the US breaks into fourteen different regions today, and 100 years from now, each region is speaking different accents of English. After even more time (100s to 1000s of years) those accents become basically new languages. One region says “fish,” and another says it like “feesh,” and over time, the words sound very different, etc. They could still have many similarities, but still be considered separate languages. This is kind of a super basic explanation, but anyway:
That’s basically what happened.
“Romance” languages like Latin, Italian, Spanish, some English, Romanian, Catalan, Portuguese all come from ancient Latin, etc — or more accurately, from the Indo-European language family.
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