My unscientific supposition.
It "appears" that people who have had COVID-19 do produce antibodies. However, it also "appears" that the level of antibody protection decreases significantly over time.
Exceeding my vaccination knowledge by miles and miles, depending on the vaccine technology used, it might it also be true that the goal is to have antibodies developed in people who receive a vaccination. If that is true, the challenge will be to develop and maintain a protective level of antibodies over time (annual?).
Or, might it be that booster shots will be required at some timed interval to keep the antibody levels at a sufficient level to provide actual protection.
Then, we also hope that the coronavirus does not mutate and require different formulations for different versions of the virus, similar to the annual flu.
When the word comes out, "We have a vaccine!" I would not celebrate too quickly. This could be very challenging time going forward.