Exactly, you want to hack a voting machine? a few key lines of code, deployed at the right time, or a shipping container full of bad hardware, and you have it. You want to disenfranchise a specific set of voters? Oh sorry some thug broke in and smashed up the voting machines last night... Suddenly voter turnout for that area is 10% of what it would have been because they had to ship in 1 machine from another location rather than the 10 they should have had.
You want to change 1% of paper votes? That means you need to buy enough materials to even create those fake votes, you need to pay off enough people to cast them, you need to spread them out geographically, you need to ensure no random person decides to sit and watch the ballot box all day, you need to ensure that not a single one of these people ever talk, etc...
Paper ballots can be cast on anything, run out of paper? Go to the corner store and buy more. Not enough pencils because of extreme turnout? Same thing. No worries about cryptographic proof, you can personally watch your ballot go into the box, and then sit there until they are counted and not let the thing out of your sight.
It's expensive because it makes breaking it that much more expensive. The manpower, the money, the time, and the chances to get caught all make paper voting extremely reliable, and it's something we as society have had a LONG time to refine and get right.