To add to Priyansh's answer, they eat seal blubber and leave the, uhm, rest, for everyone else. Polar Bears need to eat about 50,000 (yes, fifty THOUSAND) calories a day to survive, and seal blubber provides that. The calories for the rest are a rounding error, and apparently more trouble than they're worth.
Like this guy: "You want any of this? I'm stuffed!"