Honestly I'm of the opinion that clerics aren't actually that great at healing. I also don't think 'healer' is a role that is super important, though having multiple people who heals that do other useful stuff is nice.
Well they're not that great just better than the other classes.
Better than the classes that don't get heals. Bards typically heal better after a certain point (assuming they are building towards it, to a degree), druids heal better (out of combat, at least) pretty much from 1 to 20. Druids and rangers are so great at out of combat healing DMs often complain about it and want to nerf healing spirit. But even without healing spirit, using yesterday's spell slots for healing (goodberries) is just fantastic.