That and the GRV has some strict rules about it to protect them. On a PCP the consumer has LOTS of rights for handing the car back early etc and I don't think they know how the resale value of these cars is going to stack up over the next 3-4 years so don't want to put themselves at risk.
It's a concern I have (and 1 of the reasons I didn't do it this time around) - I have no idea on the life of the drive train. The batteries also are a major concern, looking at other EV's you buy the car and lease the battery (Nissan Leaf for example), because the battery has a limited life, so you can swap it out. Tesla say the battery is good for 8 year and costs ~£7k to replace, but higher milage users will have to change it earlier. Hopefully in another 2-3 years it will be clearer how this pans out!