bash doesn't have an internal mechanism to do floating point arithmetic. The $((expression)) syntax only does integer math.
You can always put a command through bc -l using the `backtick execution` syntax (or $(like this) if you prefer - as I do):
$ result=$(echo "3.14159265359 / 2" | bc -l)
$ echo $result
You could also switch shell to use zsh, which can do floating point arithmetic internally.