"Nobody knows what they're doing" is a cope
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Like when it comes to living their life as an independent adult, no one would really know what they're doing, since everyone's situations are different enough and the markers of 'success' are vague enough that there is no one answer. Or maybe politics/running a country, where again, the only real goal anyone has is 'make the world better in some way' and everyone heavily disagrees on what 'better' looks like or how it can be achieved.
On the other hand, I'd guess a pro athlete would have some idea what they're doing when it comes to their sport of choice, and a doctor would know at least something about medicine and how to treat illnesses. I don't think the saying is usually used much in these contexts.
But when I hear, "Nobody knows what they're doing", I tend to think of it more in overall terms, and given the state of the world, it's the best explanation I have for why things are the way they are - the world is at best a half-broken mess of klooges and we drift from one crisis into the next; there's no plan to any of it and ultimately no real meaning.
"Nobody knows that they are doing"
It's an easy way to discount the people that put the hard work over a long period of time to get better at something. Most people just want the results (money, fame, etc) and never want to do what it take to get those results.
I find this as disingenuous as saying that luck is what mostly contributed to a person's success. Unless it's winning the lottery, this is never the case.