It is not even about understanding. It is about how easy it is to distrust it.

Contrary to what nerds think, the goal of elections isn't to get bulletproof results by mathematical standards. The goal is to create agreeable consent among those who voted. A good election system is one where even sworn enemies can begrudgingly agree on the result.

A paper ballot system has the advantage that it can be monitored by any group that has members which have mastered the skill of object permanence and don't lie. That is not everybody, but it is much better than any hypothetical digital system

Also e-voting can be hacked (I guess they vote from their computer/smartphone, which can be hacked from the other side of the world). The last place you want to care about phishing, IMO, is voting.

Good luck hacking in-person voting or even "physical" mail voting from the other side of the world.

The ballot voting process is also misunderstood by regular citizens, even nerdy ones. From experience, even by voting officials.