This is like expecting physics, engineering, or medicine to be simple.
Law is inherently complex - partly by design, partly because of politics, but also because law is based on established concepts that are non-trivial.
Even a supposedly simple concept like "guilty" includes shades of culpability that an untrained view won't appreciate. This has a direct and very obvious effect on the level of fairness and justice available in the legal system.
So no - generally you do not want untrained people making important decisions about other people's lives. Ideally you want people with wisdom, maturity, and strong personal integrity making those decisions. If that's not available - it often isn't - then a professional knowledge of precedent and case law is the next best thing.
Lawyers are only over-educated in the sense that law is also about manipulating juries, judges, and other lawyers through drama, rhetoric, and grandstanding.
Essentially you can win a case by putting on a good show rather than winning on facts and substance. And that's not a good thing, because it has no relationship to justice.
Without that element - or at least with much less of it - lawyers would have to work harder, but the results would be fairer.