Why can't we attack two mobs at the same time?

For instance:

A small butterfly
a wood louse

# kill butterfly
...battle begins...
...battle continues...
# kill louse
You are already fighting!
use 'bladed fury' louse
...battle continues...
bf hits louse ->
NOW you can fight them both.

Why is this so? You have to use an offensive skill
to attack another mob? Why 'kill' doesn't work?