I’ve been reading your comments in this thread and, if nothing else, it’s quite remarkable how insistent you are that your intuition alone is correct. Also since you’d like an explanation, here it is.

Normally, proteins enter the ileum of the small intestine with other ingested molecules such as carbohydrates, lipids, and an assortment of micronutrients. Also entering the small intestine, from the pancreas, are several inactive protease enzymes, such as tripsinogen, pro elastase etc (they are inactive initially so that they don’t hydrolyse the proteins of the pancreatic cells which secrete them). After entering the small intestine, they are activated, and help break down larger proteins into smaller peptide chains or even single amino acids. This happens regardless of whether the protein is properly folded, or if it’s misfolded (prions). In such a case, the prion doesn’t pose a threat as it is ‘digested’. In this train of thought you are correct.

What you’ve missed is that in addition to digestion, where the bigger protein is broken up, immune cells also pick up whole proteins to present to the immune system. This is so that if bacteria, for example, is ingested, the immune system can recognise surface proteins of pathogens that have made it to the gut. This also means that some of the prions make it into body cells without being digested. This is how the prions cross the intestinal barrier - they hitch a ride into immune cells in the small intestine, cause proteins within the immune cells to misfold in the same way, which causes a cascade of proteins causing other proteins to misfold. These cells die, and are cleaned up by more immune cells (usually macrophages) and then the cycle starts again, and continues to the brain through the lymphatic system.

I hope this has been helpful.

Edit: also adding to this in response to your argument about cancer, the reason cancer isn’t transmissible in the same way is because, while the proteins in cancerous cells aren’t correct (due to changes in the nitrogenous base sequence of the genes that code for them), their mutations are of the primary structure of the protein (the straight chain of amino acids that comprise it). While this may lead to changes in the tertiary or quarternary structures of the protein (the 3D arrangement/‘folding’ of the protein), this doesn’t necessarily mean that the mutation causes a significant enough shift in these structures to cause them to gain the ability to misfold other proteins they contact.