Intention Vs Action
So you are indeed self purifying your inner self, but as humans we are created with a natural disposition to worship a higher being. It is very evident from the history of humanity that even when we do not receive the guidance of prophets, or we deviate from any such guidance, we end up inventing our own worship rituals. As humans we have an innate need to see the proof of our beliefs in our own actions. Through prophets, God has only regulated certain rituals to fulfill that need in the most effective and beneficial ways.
And this also connects to my other question regarding Allah’s directive to Prophet (pbuh) to stand in prayer most of the night in order to prepare himself for the tough circumstances he had to face during the day. How does prayer strengthen one basically?