Why not?

I don't see why one would prefer having to learn and use a second language, which they're not as familiar with as with their first language, instead of just using their first language everywhere.

Having everyone always use the same language would not just ease direct communication, it would also help making (cultural and scientific) assets more accessible, because they wouldn't need to be translated.

Most people only watch movies in their first language. If the movie happens to initially be released in a different language, they only get access to that movie at a later time, if ever.

Same for many other things. Many people post recipes they like online. Often in their first language. People who don't speak that language will never get to see those recipes. (Unless they use a translation tool to understand the recipe, which most won't do. Most will just use a recipe written in their first language.)

This hinders cultural (and scientific) exchange.

I believe that the culture that is erased by deprecating a language is insignificant when compared to the cultural exchange lost to language barriers.