We see a lot of fixed wing autonomous UAVs. What is preventing this in the case of helicopters? I know the helicopter flight dynamics are complicated. But what exactly is the complication in this case?
Multirotor autonomous craft are making headway. Here is one named Vahana that was in the news recently. The Vahana link has video too.
Here is another being tested in New Zealand https://ichef.bbci.co.uk/news/660/cpsprodpb/18433/production/_100397399_mediaitem100397398.jpg
Google "airtaxi" and there are many links to companies like Uber that have plans and visions for autonomous airtaxi service. Some have scale mockups, some have just pictures.
I haven't seen any plans for autonomous helicopters, only multirotor vehicles. The multirotor vehicles give you a chance at recovery should one motor be lost; a helicopter, not so much.
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.