Habit
It happens before you've consciously decided anything. You sit down, and your hand is already moving. There's no moment of deliberation — the reaching happens first, and any justification arrives afterward, if it bothers to show up at all.
That's not a lack of discipline. It's how habits work, and it's the same mechanism that's existed long before smartphones: a cue triggers a routine, the routine delivers a small reward, and the loop strengthens each time it fires. Sitting down on the toilet is now an extremely reliable cue — private, predictable, several times a day. Ideal conditions for a habit loop to embed itself so deeply it stops registering as a choice.
The reward side is doing most of the work. A locked screen is boring. An unlocked one is an unpredictable little machine — sometimes something, sometimes nothing, sometimes something worth having seen — and unpredictable rewards are, reliably, the most habit-forming kind available. No guarantee is required. Just the possibility.
This is why "I'll just not check it" tends to fail. You are not fighting a decision. You are fighting a loop that fires automatically, faster than the part of your brain doing the deciding gets involved. By the time you notice you're scrolling, the loop has already run its course.
The one part you can actually control without relying on willpower is the cue. If the phone isn't there when you sit down, there's nothing for the reach to close around, and the loop never starts. That's the entire logic of keeping your phone physically separate from the bathroom rather than mentally resisting it — you're not out-willing a habit loop, you're removing the object it depends on.
And that's why I urge you, for the sake of the human race and your own thumb's autonomy, to consider any product that removes the object before the loop gets a chance to run. We happen to make one — drop it. Your hand will thank you, eventually, once it stops looking for something that isn't there.