I ended up on the other end of this list: I built the document scanner because I got tired of looking.
It took a weekend, because the actual work is a camera preview, edge detection, a perspective transform and a PDF writer, and all four of those are library calls now. It has no ads and no account because it has no server and no costs.
The reason nobody sells this properly is not difficulty. It is distribution: a scanner with no marketing budget is invisible next to one that can buy the search term, and the one buying the search term needs revenue per install to justify it, which means ads or a subscription. The economics select for the thing you are complaining about.
Which is a depressing answer to "why does this not exist": it does exist, repeatedly, and you cannot find it.