Imagine that you received the instruction to see and look at a blue dot on your wall and nothing else.
How you would stress and strain to accomplish this task. You might even catch glimpses of 'success', for, through force of will, one can in fact stop the saccading of the eye, whereupon they will find the whole of the vision, starting with the edges, turn to a featureless grey. You'd find your eyes straining, growing dry as you fail to blink, your brow furrowed as you tense to maintain this state. And you'd make little progress, even after a year or more...
If instead you were instructed: just keep the blue dot mostly centered in your vision, and don't try to interrupt the normal processes of the eye, small movements, blinking, some expansion and contraction in how much you notice the periphery. You can imagine finding success, though it would still not be easy, to stay with the blue dot for increasing lengths of time as you notice all the variation in activity that goes into staying with the blue dot. What is blinking really like? The tendency of the eyes to relax and focus more sharply, the intrusion of the peripheral vision into consciousness to greater or lesser degrees, the coming and going of desire to look elsewhere...and insight would occur and progress would be made.
So too with the breath.
I find myself frequently reminding people that the other translation of "Samatha" is 'tranquillity'. That to try to unify the mind via force is much like trying to hold on to a handful of jelly by squeezing.
(this post brought to you by the blue dot on the bottle of soap visible from my computer)