...Your mistakes are their mistakes. If you screw up, they may lose money, they may lose custody of their children, control of their business, their freedom, their life. Although your mistakes are your client's mistakes as well, if you screw up, they can come after you...legal advice is cheaper, but also that legal advice is of a poorer quality and less useful. It also means that unqualified people will be dispensing legal advice with real, detrimental, consequences to the lives of people that cannot afford quality legal advice....
And how does that malpractice against a lawyer work exactly? How rich does someone have to be to absorb your fees, the lost lawsuit, then float the losses and second lawyers costs for years through appeals? Why can't the cheaper "lower quality" provider be sued for incompetence just the same as you?
The implication of your argument is that those that can't afford quality legal advice should have no legal advice. WebMD and/or Google is good for the vast majority of cases. LegalZoom type services are also good for the vast majority of everyday cases. You need to see a doctor/lawyer if you have something that is complicated or uncommon. For those that can't afford the doctor/lawyer, how does removing the legalzoom/WebMD/Google options make them better off?
No amount of careful planning will ever replace dumb luck.