PDF was basically created as a final presentation format, it's essentially append-only by design, which is why you hear so often of redactions just marking up a white/black box over text.

You can edit text streams, which needs a decompress/recompress, messes up all the object reference offsets for the entire file, potentially adds or removes characters to the subset font, which may not be referenced by the glyphs but a number instead to intentionally break copy/paste/scrape (e.g. 'a' is not 'a' in the text stream, but a random number), etc. Assuming the text is even marked up as strings, and not individually positioned characters with nested offset co-ordinate scaling to further muddy the waters.

The fact so many people want to edit PDFs probably indicates a design flaw on Adobe's side, when considering what customers really want.