"Is it because of the sheer difficulty of such ventures?" Keeping R&D alive is difficult, especially in a competitive market. You need to keep momentum going.

"Is it because of the sheer difficulty of such ventures?" Keeping R&D alive is difficult, especially in a competitive market. You need to keep momentum going. One way to help success rate is through economies of scale. If you know or have a hunch you can sell huge volumes, then you can more safely put money towards the infrastructure that will bring down cost per unit - whether that's for a full vehicle, or individual parts like batteries for a vehicle.