People can be allergic to anything, and penicillin is pretty much mold cultures. The fact that it is used as an antibiotic doesn't make it more or less likely to be an allergen.

One thing to add is that many people were told they are allergic to penicillin by their parents as a result of developing a rash after its use as a child. Often this rash was a result of the infection itself or from toxins released by the bacteria when they were killed and not actually an allergy.

Penicillin is not the "pinnacle" antibiotic at all. It was the first one discovered. Since then thousands more have been created, most of which are better for at least a few types of infection.

And all allergies have the same root cause: some molecule is similar enough in structure to a molecule we use in our own body that our immune system gets "confused" and attacks us thinking it is attacking invaders.

Penicillin and its derivatives have been around longest, and are therefore the least expensive and most widely available. The more people take them, and the more frequently the same person takes them, the more opportunities there are to develop an allergy.