Because it’s the same language. I grew up in Argentina, and the “Spanish” (the name of the language is actually Castilian because there are multiple languages in Spain) we learn at school is the “Spain” one. In reality it’s the language as defined by the Real Academia Española so the language is the same (yes it includes the vosotros conjugation, no, no one outside Spain actually uses that but we learn it in school).

The differences between Mexican, Argentinian or Spanish Castilian is more in the pronunciation and the use of some words, but the language we learn at school is all the same, and I imagine it’s the same one that you learn too.

That being said, using vosotros to us sounds similar to how using thy might sound in English. A good teacher would explain that outside of Spain we use ustedes which uses the plural third person conjugation (i.e. the same one as ellos), but the correct plural second person is vosotros.