Having it and recovering from it still occasionally leaves your body altered/damaged in a way we're not sure how to predict/treat. As far as we can tell, the vaccine does not cause the same amount of damage that actually suffering from the disease can cause.

Getting it early and getting immunity works great for fairly mild things like chicken pox, which is less likely to leave permanent/long term damage. But not for something where so many people report that they've still never fully recovered from even months later.