Because I don't have a double-yellow line painted between my property and my neighbors? Machine learning is fine and all, but you need something for the machine to learn from. Without some kind of outside influence, my grass looks the same as the grass across the invisible property line.
Not only that, but you need a system to account for wheel slippage, etc. No matter how accurate your compass, how awesome your internal map, you're eventually going to go off course. Best case, your bot mows your neighbor's property. Worst case, it wanders into the busy street, or into your pond. Systems like these beacons or underground wire help ensure this doesn't happen.
Source: I have a half-built robot lawnmower on my workbench.
It doesn't steer. It augments the cruise control, accelerating and decelerating in stop-and-go traffic (see https://www.youtube.com/watch?v=PcwObeGB_mU). Which isn't nothing, but it's not driving you to work.
I've heard them claiming that it could indeed drive you from home to work using public roads and that it is capable of finding you and driving the car to you when you call for it (aka. "Batmobile mode").
Hm, I didn't know that. (Edit: it doesn't seem to be out yet?) And some cars can steer around corners on the highway. But it's very limited, doesn't necessarily work in the dark or in bad weather - and most relevantly it doesn't actually navigate by itself. It just follows marked roads. To use the self-driving lawnmower, you have to mark out an area for it to mow.
Which planet would that be? I see them nearly every day. They certainly aren't perfect, but I think it's reasonable to say that we have working prototypes of self-driving cars.
How are you going to know where someone's property ends? You're not going to equip a lawn mower with a mounted laser range finder and sophisticated image processing capabilities.
Not only that, but you need a system to account for wheel slippage, etc. No matter how accurate your compass, how awesome your internal map, you're eventually going to go off course. Best case, your bot mows your neighbor's property. Worst case, it wanders into the busy street, or into your pond. Systems like these beacons or underground wire help ensure this doesn't happen.
Source: I have a half-built robot lawnmower on my workbench.