Muscles are pretty amazing. They have a higher strength to weight ratio than pretty much any small actuators we have. That strength is essential for smooth dynamic movement (the forces you encounter trying to pick up a gallon of milk, open a jar, get yourself out of bed are surprisingly large). In addition we don't just have muscles that go forward and back, or up and down, like many actuators. We have dozens of muscles engaged during pretty much any task that allow for flexible 3D force application. This is then coupled with reflexes and the brain's ability to accurately predict the body's motion, and that of things you interact with. Robot actuators are almost all reactive making their processing speed a limitation for control.* Humans all use long range predictive control to apply forces preemptively to smooth out motions. Finally we're highly optimized for an effort minimization, we don't just choose motions that work, we choose motions that are efficient. That objective and ability goes all the way back to evolutionary influences.

So yeah, smooth motion feels easy, but is a gd miracle of biology :)

* Robots can make up for a lack of prediction through really really fast control. This is how Boston Dynamics robots operate at a basic level.