Why Is Wage Growth So Slow?

Because it can be since stuff employees might want to buy didn't become significantly more expensive as a sum.

People on average earn only as little as they can because there's no point for the employers to pay them more.

Why has wage growth been slow for thirty years now?

Employers are paying more to retain workers, but at least in the US that extra money is getting soaked up by the health care system instead of ending up in peoples' paychecks.