Here's why: We can experience only sensations that change. Anything that doesn't change rapidly fades from our awareness. If we can perceive it, it's changing, and that's a kind of "doing".
Things that change are all "doings"; even inanimate objects are doing something, if only capturing and re-radiating ("reflecting") light, and that light creates a change in our experience that makes it possible to perceive something.
The notion that being and doing are separate and different leads, in those who seek peace or other spiritual attributes, to seeking for being -- a kind of doing. They abandon "already being" for seeking being -- a kind of doing!
That being the case, all seeking for "being" or "peace" or other spiritual attributes is fruitless. Not just frustrating. Fruitless.
Now, oddly, the act of seeking is the action of "being, seeking".
All actions come from an unfathomable depth in ourselves. That unfathomable depth is our being; it is "the seer which is unseen". (If it can be seen, it's an object, not the seer.) Doing emerges from being. Something comes from nothing. (Where do you think the Universe came from? -- or if you have a theistic view, What do you think God's nature was, before creation?) Doing is the action of being, while being remains forever unseen, only the doing being perceptible.
In reality, we can never depart from being, nor can we be separate from it -- mainly because there is no "it"; all "its" emerge from, are pervaded by, are "made from" and abide within "it".
So, don't worry about "achieving being"; you can only achieve "doing" anyway, which is the action of being.
The way to do is to be.
-- Lao Tzu
Do-Be Do-Be Do
-- Frank Sinatra