Leaving aside, for a moment, the death penalty’s dubious deterrent effect, the more immediate issue with “automatic” capital punishment is that it’s illegal.
If a defendant is convicted in a death penalty case, then a separate trial is held in which the jury weighs aggravating and mitigating factors, to determine whether to recommend a death sentence. Prosecutors wouldn’t go through that process if they didn’t have to. Indeed, the Supreme Court has struck down mandatory capital punishment on multiple occasions, reiterating that individualized sentencing is needed.