Elections with paper ballots are somewhat straightforward in that regard. Any party member that doesn't trust the process can literally apply to check part of it and be able to see for themselves that there is nothing fishy going on in the part they checked. And they can do that with nearly no prior expertise. If they don't get to check it for themselves they can trust that enough people like them are envolved in the process that someone would collect evidence of wrongdoing if it happened. And these people are normal people and many, so bribing them doesn't make sense. Adversaries that want to make that election untrustworthy would have to insert so many people at so many steps and at such number, an attack against it quickly becomes impractical.
Not with a computer system, I am not even sure if I would trust a system that I myself setup and software that I myself had written if used in a high stakes election. But the few experts that are able to verify the process and have the computer knowledge to do so without naive optimism now are high stakes targets and each party now needs to have one at each polling station at some point after which the machines (air gapped?) need to be completely isolated unless you want that verification to become meaningless. The voter now needs to trust the expert and for a hint about how well that works I want to point you to the Covid pandemic.
So digital voting is a non-trivial problem to solve, especially for high stakes, anonymous, but transparent elections. And we computer people can't just hand-wave the doubts away, you need to address each attack vector a major nation state attacker could/would exploit. And even if we did that the result would be a system that nobody without special education could understand.
Sure, paper ballots are slow and the process lengthy and labour intensive. But the results are surprisingly stable and trustworthy even in many places where one would expect corruption and manipulation.