For the same reason presidents, geometric shapes, and musical instrument appear in food. A random shape is seen and our brains link it with something we are familiar with (just like seeing shapes in the clouds).
The problem is, it usually seems to be a fanatical person (someone who is hoping and expecting to see signs (check out Mark 13;21,22 and 2 Thessalonians 2:9)) that sees the image first and makes a big deal of it.
Since they find some faux importance to a shape that resembles our Savior, the tell everyone about it. That is why it seems that Jesus and Mary are the only images that are ever seen in food.
The search for images that look like people they have NEVER seen only detracts from the message that Jesus taught. Since this is the case, we must realize that the images are either mere coincidence or, at the extreme other end of the spectrum, possibly works of the devil. Jesus NEVER did anything that detracted from his message of God's kingdom. But that's all these images do.
If enough people thought that President Washington could intercede in their affairs, or that Yul Brenner might bestow a blessing on their household, we'd hear about all the times their resemblance was found in a pancake or water stain. As it is, the only time we see those are when we are watching America's Funniest Home Videos.
It's a matter of perception and dissemination. Because people want to see something, their minds constantly look for it in a subconscious way. When they find it, they tell everyone that will listen, even news reporters who are having a soft news day.
That's why it seems that Jesus is the only one to constantly appear in food.