A “shitty first draft” is how writers begin their work—not by writing a perfect draft from the get go, but by literally dumping their thoughts into a draft that gets iteratively improved.
Very few writers really know what they are doing until they've done it.
It is not that writers can’t write a better first draft, but personally I know that having any expectation of quality on the first few drafts will make the writer self-conscious, impeding flow. Simply writing your thoughts may even lead to more interesting ideas that “you would never have gotten to by more rational, grown-up means” (@lamottShittyFirstDrafts1994). Just write first, clean up later.