Because biology is complicated and it's never that simple. You could trigger too strong an immune response, no response at all, or you could end up with a situa

Because biology is complicated and it's never that simple. You could trigger too strong an immune response, no response at all, or you could end up with a situation like on dengue where the vaccine makes the disease worse (https://www.statnews.com/2019/05/01/fda-dengue-vaccine-restrictions/ https://www.statnews.com/2019/05/01/fda-dengue-vaccine-restr...)