Animals in cages on factory farms or in laboratories don't suffer that much because they've never known anything else. To be prevented from performing the most basic instinctual behaviors causes tremendous suffering. Even animals caged since birth feel the need to move around, groom themselves, stretch their limbs or wings, and exercise. Herd animals and flock animals become distressed when they are made to live in isolation or when they are put in groups too large for them to be able to recognize other members. If animal exploitation were wrong, it would be illegal.
Legality is no guarantee of morality.
Who does and doesn't have legal rights is determined merely by the opinion of today's legislators. The law changes as public opinion or political motivations change, but ethics are not so arbitrary. No, but enough people have filmed inside and written about what goes on in these places to tell the story. You do not need to experience the abuse of animals close up to be able to criticize it any more than you need to personally experience rape or child abuse to criticize those. No one will ever be witness to all the suffering in the world, but that doesn't mean we shouldn't try to stop it.