Of course, it's not possible for literally every citizen to this themselves, but since citizens already have to trust the party they are voting for to some extent (otherwise, why care about the election?), this works out very well in practice, especially if you add a few citizens organizations and international organizations in the mix monitoring as well.

Electronic voting is orders of magnitude more complex, even in simple centralized schemes. In more decentralized schemes, it becomes completely, hopelessly intractable to ensure validity unless you chose to blindly trust hardware and base software (microcode, BIOS, disk firmware etc.). And the same argument about trust above doesn't apply here, as it's not your preferred party's representatives that have created the hardware etc.

This is why electronic voting is unconstitutional in some jurisdictions - Germany, at least.