The primary reason I say that is to encourage people to do the work to understand how their very actions ensure it. The deeper down the rabbit hole you go, the more you know this and the more you tune out the noise.
I believe/know it stays decentralized and secure because I am one node (my thoughts, words, actions) in keeping it that way.
And because that is my frame of reference, I see how many others are doing the same.
We are bitcoin