Why do we not have autonomous helicopters?
Don't we? Most consumer and commercial UAV are helicopters.

If you mean military-only models, consider what jobs military-only helos do. It's troop transport or close combat - both necessarily manned functions. Commercial drones suffice for non-military-specific jobs.

If you wonder why they have 4 rotors rather than 1-2, it's lower complexity - quads can use simple fixed rotors and blades, and only vary power on each rotor. Larger drones with turbine power may well go to 1-2 rotors.