The simple exercise of watching one's breath is by no means easy, even for 5 minutes. All our lives we've allowed our attention to turn to anything that pops up in the outer or inner world.
This is like trying to carefully observe an area of forest during a dark night with a floodlight, but the floodlight is loosely swirling around with the prevailing winds - frustrating!
Persistently bringing the focus of our attention back to eg the feel of our breath in the belly, retrains our brains for stable awareness. We gradually learn to hold the floodlight, stabilizing it on whatever we plan to observe.
It's like learning to play a musical instrument for the first time. There's a learning curve, but as we all know, the more one practices with full intention (not just go through the motions), the better one becomes at a new skill.
It's simple training effect - neuroplasticity - the brain rewires itself, and the activity becomes progressively more natural, easier to do.
If you feel that stable awareness is a fundamental life skill, then it's well worth the persistent effort to cultivate and refine.