Underneath a Tesla, and I imagine other EVs with similarly located batteries, there is a titanium plate. It is there to prevent road debris from penetrating the batteries and starting a fire, but it is precisely the sort of thing people install to prevent catalytic converter theft.
This is setting aside a larger problem: If you're under a car nabbing a catalytic converter you can quickly cut it loose, grab it, and run. If you're under en EV and manage to break the battery pack loose after several hours work, your remains will be pinned to the ground under that car later when the police come to investigate. People won't be stealing batteries and leaving EVs stranded for the same reason they aren't stealing diesel truck motors and leaving the trucks stranded.
This is setting aside a larger problem: If you're under a car nabbing a catalytic converter you can quickly cut it loose, grab it, and run. If you're under en EV and manage to break the battery pack loose after several hours work, your remains will be pinned to the ground under that car later when the police come to investigate. People won't be stealing batteries and leaving EVs stranded for the same reason they aren't stealing diesel truck motors and leaving the trucks stranded.