Think about it this way. Even when you sit completely still, your heart beats, your lungs breathe, your mind races from one thought to the next. Complete inaction is a fantasy. Every living being, at every moment, is performing some kind of action. Krishna states this as a plain fact.
And the reason is prakriti. The three gunas — sattva, rajas, and tamas — are always at work inside every person. They drive action whether you notice it or not. Someone who sits still and claims to be doing nothing is simply unaware that the mind inside is running at full speed.