Mumma Bear eating

Because being dangerous has nothing to do with whether humans consider something cute or not. For example, the Slow Loris:

It is the only venomous primate, and it coats its young in its venom, which makes them both venomous and poisonous. But they're extremely cute, aren't they?

People consider polar bear cubs cute because they are small, fluffy, and behave like something young. They aren't as coordinated as their adult counterparts, which is funny and cute in human eyes.

Adult polar bears are fluffy, which humans generally consider cute. They also act like humans in some ways, but since they obviously aren't as good as it as humans, it is considered funny and cute. They also just sit there, which is funny, and therefore cute to humans.

Basically, they do human things, are fluffy, and aren't good at human things. That's a recipe for cute if I've ever seen one.