when someone asks me sth and I want to answer “why not” what would be best thing to say?
You mean “why not” as in “I don’t know, I just felt like it”? If so, なんとなく might do the trick.
yeah I meant sth like this. なんとなく sounds right though
A: why the heck is your phone in Japanese
B: why not 
Depends on what you want to convey - なんとなくas seanblue said works fine and is pretty non-committal, or if you want to be more affirmative, you can say 別にいいじゃない
*edit: fixed typo
true, that’s what I also hear alot too when I watch different shows. thank you
じゃない.
気を付けてね
Ooh thanks for catching my typo!