Though I say the "passive" "active" distinction isn't all that useful. When you are sitting still and breathing, your are actively observing the sensations. There's lots to see.
Lying, sitting, standing, and walking, are all different vehicles for practicing mindfulness. For some people, walking is too stimulating and need to sit. For others, sitting is too restful and one easily falls asleep.
I've mentioned this elsethread. Formal practice where you set aside time and make a commitment creates a ritual container for the practice. It creates a timebox from which you can bind your resolution and intent to practice. You can make this more or less elaborate -- and some people need elaborate.
The formal practice is to give you a chance to realize insights which you then integrate outside the timebox. As the saying goes, mindfulness doesn't end on the cushion.
So if you're able to be mindful in all the things you do, that's awesome. There are lots of people who are like you. And there are lots of people who need a more formal container for their practice.