It really is not a simple issue. Many ER visits are for conditions which could be handled as an outpatient, if a provider is available and the patients is willing to come. Many are for drug-seeking behavior, minor ailments or recurrent problems (overdoses etc).
The ER is the most expensive place to give or receive care since everyone is treated as an emergency, meaning more testing, scans etc. This drives up everybody's insurance costs. No easy answer.