When paper vaccine certificates were introduced and became formally recognised, photocopiers were very expensive and reproduction quality was cr*p. (I worked in a print/photocopy shop in the early 1980s)
Colour laser printers are cheap and readily found in people's homes. The image quality is excellent. The reason the UK moved to polymer banknote was because it became impossible for the average human to spot fake banknotes - even the hologram was often pretty convincing
Rich countries do not generally like passports which do not contain a chip - this makes them difficult to forge. Immigration policies sometimes make entry more difficult for passports without a chip
Rich countries will accept vaccine passports for immigration purposes only if they are difficult to forge. You should expect quarantine arrival policies to make entry *much* more difficult for vaccine certificates that are easy to forge