I agree completely. Taking a shower, brushing my teeth and other routine things have never become a habit for me either. I have to put in the same effort every time.
Of course there are habitual things in my life, but I can see no common characteristics. Total time spend certainly isn't it.
Amusingly, this has made it almost trivial for me to quit smoking despite being a smoker for many years. I just never developed a habit for it, so dropping it was as simple as just not smoking.
That's what the entire internet tells me, but it just doesn't happen to me.
Whatever it is that I wish to keep up, I have to decide to do it every time, roughly the same as I did the first couple of times.
Yes, that includes brushing the teeth.