Having prepared and eaten many, I don't doubt that sandwiches exist, even if only as ephemeral things.
For me, if something is tangible to my senses I will almost always believe it, mirages and illusions excepted.
If there's anything to doubt about sandwiches, it might be whether they are the most convenient lunch to take to work or on hiking trips. They need a proper container.
Or one could debate the nutritional value depending on the ingredients used.