1. 20% require hospitalization. Hospital capacity is limited.

2. It's questionable whether we can become immune to it. We might keep getting re-infected.

3. It causes permanent lung damage in some patients. Makes re-infection even more lethal.

4. There's a chance that animals can catch it and become a reservoir for it. Imagine if every dog, cat, and pigeon in the world is infected with it, but doesn't die from it.

Yes, it's low probability. But it's not zero. And it's not negligible either.