Fruits contain seeds because that's how their trees reproduce. An apple falls off the tree, the seeds get buried, and a new tree grows. But banana trees (actually giant herbaceous flowering plants) work differently. Every season, the plant dies after its fruit is harvested, and the small bulbs (called the suckers) growing out of the plant's underground rhizome (called the corn) are then replanted, and new plants grow. Put simply, bananas don't have seeds because they don't need them.
Because all bananas have been propagated vegetatively (as this process is called), all bananas are sterile clones, and just about all of the bananas you find in the grocery store are a single breed, Cavendish. The quality is consistent because they're all genetically identical, but that also makes them very prone to disease and parasites as resistance cannot be bred into them. So the banana breed that was predominant 50 years ago, called the Gros Michel, is all but extinct now because of a fungus. And though the Cavendish is naturally resistant to this fungus, word is that they don't taste nearly as good as the Gros Michel.