Because you cannot verify your phone is not compromised at either a software or hardware level. You would need independently verifiable hardware and all softwa

Because you cannot verify your phone is not compromised at either a software or hardware level. You would need independently verifiable hardware and all software running on a closed system (ie, no third party modifications to running software which would mean at most a trusted sandbox for other applications outside the proven path) to be able to trust it to reliably take your vote. Thats on the order of correctness provability that NASA puts into launch vehicles but NASA doesn't have to contend with hostile actors seeking to undermine their software and hardware.