Well, the way I see it, I wouldn't say so much like that. For me the first page is used for other things. To begin with, the first sentence is the hook and the rest of the first paragraph is the development of that, which can be in many ways; for example, starting with your foot on the accelerator, in such a way that that first paragraph can be more or less long or even cover more than one page, although this technique corresponds to an overlapping of one sentence after another, usually using a lot of comma or semicolon to avoid interruption and whose objective is precisely not to give the reader respite. In boxing terms, one punch after another. The difficult thing, yes, is to find what to say, which is why it is recommended for some cases and not for others, it all depends on what is being told.
An opposite case, for example, is the slow wick, according to which you gradually gain speed similar to a wave, that is, you guide the reader through an ascent, climax and its corresponding descent in each chapter. But again it depends on the type of story. Even so, it is often said that there are writers who are gradually gaining strength.
I personally don't see an obligation that the subject should be explained on the first page. You can give clues , yes, because the reader begins reading wondering what it is about, so, at least in the first five pages, I would say, you cannot allow yourself to digress; that's for other chapters, when you've already accomplished certain goals like setting the narrative tone; to present not necessarily the MC but some liaison character, although, of course, it would be appreciated if the idea of the story was expressed within that first chapter.
