Heavy bike, lots of torque, soft tires, that's actually a little better than I see some post, and better than I get some of the time. For a time I made it a point to not ride like it was a 650 got 12,000 on a rear tire. But that's no fun.
For what it's worth I see some Harley owners talking about the same tire mileage.
A motorcycle tire with a flat tread is not for me. If you check around you might find a tire with a harder tread.