Because getting infected still carries a significant risk of illness, permanent health problems, or death with it, especially for at-risk groups. Getting vaccinated would sidestep that risk.

And even if everybody did recover, there's the economic cost of an infection. Productivity missed because workers are sick at home, or worse in a hospital being cared for.

I'm not sure what the dollar figures are, but I would not be surprised if vaccinating a large population turns out to be significantly cheaper than having a small percentage of that large population in ICUs for weeks on end.