Because many automated systems/image hosting website (imgur, etc) will process JPEGs to optimize them, which I assume would no longer cause there to be a collis

Because many automated systems/image hosting website (imgur, etc) will process JPEGs to optimize them, which I assume would no longer cause there to be a collision. On the other hand, PDFs are almost universally sent as-is, increasing the risk of being able to 'fool' someone.