These are a lot of questions. I don't have all the answers and my answers might not be right but .. well I can answer questions. So....
It might be called assumptive writing but I've heard it described more often as in media res
. It's a term from Greek theater and means pretty much what you'd assume it means. But let me look up the exact definition.
It means: Into the middle of things.
Why do we do this, and why do we feel it would be wrong to put our cards on the table?
There are probably lots of reasons for doing it but I prefer writers do it this way, most of the time, because when they don't, I feel like I'm being taught something rather than experiencing something. Fiction readers don't read to learn. At least, they don't read to learn what the author wants to teach them. They read to experience or feel something. Which is a different type of learning but the distinction matters.
Wrong might be an overreach. It's not immoral. Mostly, it's ineffective. Even authors who aren't writing suspense want to create an experience where the reader wants to know more, feels there's more to know, and wants to turn the page to find out. If we lay all our cards on the table, they know what's going to happen. Mostly. There isn't that much variation in human behavior or plausible ways in which fictional characters might behave.
I don't think its fear or egotism. Over explaining is fear of not being understood and/or a lack of confidence. It can often come across as contemptuous to a reader. My thought is often, "I don't need to know this. Why am I being authorsplained to?"
Not telling the reader everything doesn't mean that the USP has to be revealed later.
I do have to admit something. Although, I don't think this is common. There have been times when I've held things back because I am uncomfortable with people knowing what my story is about. I don't want to be seen. The feedback I get from these stories is that what I've written isn't clear. I don't get told very often that I have too much exposition, but the opposite. Not that I don't have things I've written with too much exposition ... I do. But I wouldn't characterize what I do ... or have done ...in hiding my story as something people 'usually' do. It's a personal problem, not a writer problem. I'm pretty sure the writing was at least adequate even though the plot was indiscernible. I don't have that 'feeling' anymore. It's a feeling of shame and was something which impacted everything I did, not just writing. Writing has been one of the few things I could do which would provide relief from that feeling. But then I didn't want people to read what I'd written. While this is fear, it's not fear that my ideas aren't good but more of a general unspecified fear.
I think it's a good idea we don't throw writers into one universal heap or confuse good writing craft with what are personal issues which exist independent of writing and actually have nothing to do with craft.
Also, anything done without a darn good reason isn't worth doing. An exciting scene at the beginning of the book, a car crash or a murder, which turns out not to be important and something which was inserted simply to add a little fireworks, isn't what in media res means. You can put people in the middle of things without explosions or melodrama. It's a kindness to your reader because it shows a little respect for their intelligence and ability to figure out what's going on all by themselves.
What you describe Agent Pete saying sounds more like a dark room with a spotlight. Which meant (to me) that it's a good idea to use point of view and a strong voice to direct the reader's attention. I once read a gothic American story. The author painstakingly described her main characters walk across the room and all the items which she passed.
But as a reader, I don't care what's in that room unless what's in the room is significant. I have enough imagination to know what a living room in an Eastern bed and breakfast looks like. I don't need every detail described. The emotional part of that scene was when the character rifled through the papers on the table. She'd lost something. Yet the majority of her words were used to walk across the room. If you're in a dark room, with a spotlight, you direct the readers attention. The character walks across the room. She's faced with a table full of maps and papers. But something not right. What is it? She frantically searches through them, pieces of the paper dropping to the floor... she climbs beneath the table ...
Whatever. Her words needed to be devoted to that part of the scene... describe the papers on the desk ... not the chair in the living room ... at least not unless that chair starts to walk. That might be interesting.
I may have said things people have already said because I wrote part of this ... left my house ... and then came back. So ... if I'm repeating what everyone else has already said ... my apologies.