I don't think they're actually harder. Stiffer construction perhaps. If the rubber compound itself was harder, you'd be sacrificing grip, which I don't think you'd want with a heavier vehicle with instant torque up the wazoo.

Quick googling session tells me that, on average, EV tires are a little bit more expensive (~10-20%) due to more exotic compounds used and last a little bit less (also about 10-20%) due to the weight, but they still have fairly soft compounds to retain good grip, which is good news because safety is the last thing you should sacrifice.

At the end of the day, each tire is a compromise, and you simply can't get similar grip AND better wear AND better efficiency as an existing high-end tire unless you find some truly super exotic compounds, in which case it would have to go up in cost quite a bit.

There are also different compounds for different applications, I very much doubt a Nissan Leaf's tire is comparable to a Model S P100D so the Leaf may very well have harder tires for better wear and efficiency, but unlike the P100D, you won't be comparing it to a super soft sporty driving focused ICE tire either, you'd be comparing it with economy tires in an ICE vehicle that will also be somewhat hard.

Don't teslas ship with perelli's? Those are super soft tires...they grip really well but with the torque those things throw I think they'd last half as long...