They talked about this in one of the movies
People were using the night as an all night long party with no restrictions, then the government and certain wealthy people employed organized kill teams to go out and start massacring people.
Same thing would happen. Most of the lone nuts who wanna go out and kill would get sniped by homeowners+good samaritans. The real murderers would be get paid to operate like a military unit and unleash death.
The point of the purge was to unleash humans primal desires so that crime is eliminated for the rest of the year. Can't do that if everyone wants to have fun, but if we kill your daughter son and wife this year, guaranteed next year you will be doing some killings.