Not knowing things is perfectly normal. In fact, every human will inevitably go through life not knowing even a fraction of all there is to know. Publicly admitting what you don't know is often the first step of learning. It's commendable, but not all of the time. Sometimes admitting ignorance is not so praiseworthy.
Here are some examples I got from Twitter:
They don't always have to be negative. They can be good or even neutral:
i don't know why but i really love it when i'm staring at you when you're not looking.— ㅤㅤㅤ (@chrzlxpsc) January 24, 2018
I don't know why but i actually feel sad as shit if i kill a bug or something, like that lil dude just going bout his day then outta nowhere hes getting stomped to death— Maltz (@TheMaltzAssault) January 26, 2018
Notice that those sentences would work without the "I don't know why, but." So why do people say it? How do they benefit by adding that phrase?It may seem trivial, but I think that phrase says something important about the communicator.
Let's face it, they do know why.
One reason that they say it anyway could be to deflect responsibility. In other words, it's a way of removing themselves from what they are about to say because they are conveying it's not something they are in control of.
Maybe they are trying to preemptively diminish what they're about to say so that they don't sound as mean, weird, silly, arrogant etc.
The reason why this phrase interests me is that I've caught myself saying it before, but over time I stopped saying it altogether. Instead, when I feel myself almost saying it, I stop and contemplate. I think to myself "do I really not know 'why' or am I trying to diminish my responsibility?" If it's the latter then I just remove the phrase. If it's the former then I think of it as a great opportunity to find out why I think or feel the way I do.
Unless you genuinely don't know "why", just don't start a sentence with that phrase. Don't hide behind feigned ignorance - take full responsibility for your thoughts and feelings. And if you don't feel comfortable saying something without that phrase then maybe you shouldn't say it.