I'd guess it's the fact that nothing supports editing PDFs except Adobe Acrobat. Not to any sufficient extent. LibreOffice Draw kinda try to do it but also corrupts the file IME.
Edit: Apparently some people can edit PDFs reliably with MacOS Preview.app, LibreOffice Draw or pdftk.
Edit: Apparently some people can edit PDFs reliably with MacOS Preview.app, LibreOffice Draw or pdftk.