Paper and pencil doesn’t work in the US because of how litigious we are, and because each state operates their own elections. If every state ran elections the same way we could standardize on something like the UK has and have collective faith in it.
When some battleground states routinely come down to less than 1% of voters, incredibly marginal effects matter.
The US still has institutional trauma from Hanging chads.
Just look at all the varied reasons mailed ballots are rejected, especially in tight races.
With paper and pencil, what happens if a mark got erased? If the circle is half filled? If the circle is quarter filled? All of these are questions that will be the subject of lawsuits in a mix of jurisdictions.