Languages aren't some sort of noble end unto themselves, other than study for linguists.
Language is for communication. If you can adequately communicate your thoughts in a "living" language, what do others matter?
That's like asking "why paint if you have a camera?" or "why make pictures in color if black & white will do?"
Language affects how - and even whether - certain ideas can be expressed (see the appendix to "1984"). At an extreme, one can expand one's own thought process by adopting another language, improving the toolset for conceiving and expressing ideas - even if only within one's own head. The notion of "can you adequately communicate your thoughts" is subject to self-restriction derived from not knowing if there is a superior form of communication, and not knowing if you're not thinking something because you don't have the semantic toolset needed to formulate ceratin thoughts.
There's a reason why Italian is preferred for opera, English for technology, Korean for literacy, Japanese & Chinese for calligraphy, French for romance, C++ for systems programming, Lisp for AI, etc. - each is better suited for certain categories of cognitive expression. There are great works written in dead languages ... beautiful expressions of notions which are lost because nobody understands them; more's the pity.