Because those off-the-shelf solutions only work for very obvious and basic cases, and you don't know what you don't know. Meaning, if you have a more complex medical situation, you wouldn't be able to tell, so you wouldn't know if the off-the-shelf solution is wrong.

Also, we still need doctors to perform physical tasks, like surgery. The doctors diagnosing stuff and the doctors performing surgery aren't different doctors, usually. When I got diagnosed with Testicular cancer, the urologist who felt my balls up and said "yeah, this ain't right" was also the one who removed the testicle. And, he wasn't the first doctor I went to - the other doctor clearly had not felt up enough balls. He said it could be X, could be Y, maybe Z. Not the urologist, he knew right away. So, I think it's more complex.