Unless you inspect every voting machine, smart-card reader, internet connection, encryption algorithm, etc. for every election completely, you can potentially manipule millions of votes from your home from the other side of the planet. Or on a smaller scale, change all votes of one or multiple voting machines.
You don't need to inspect every internet connection, there's HTTPS.
The key is that one should be able to verify one's vote to ensure that it was properly recorded. If everyone can verify their own vote, then they can verify that the voting wasn't hacked. With paper voting, it is impossible to verify your own vote - it is equivalent to tossing it into a black box.
One might say "then people can sell votes!", but as Estonia has done, you can have it so that one can vote multiple times. And of course there should be extraordinarily stiff penalties on anyone buying/selling votes (I'd say minimum $30k or 20% of one's total wealth fine + prison time).
Needing paper ballots is holding back democracy. E-voting would revolutionize democracy and for the first time in history allow a true direct democracy at scale. Of course e-voting cannot be conducted by emailing votes or some other insecure nonsense like that, but it is ludditism to suggest that it cannot be achieved, which many seem to suggest. This kind of thinking is literally holding society back.
Each reporting station must accept observers and counters authorised by each of the interested parties with local representation. It's a big deal when this process is compromised and always makes the news. And these counters and observes are generally your neighbours from the local community so there is some amount of trust already. Further the section count is acknowledged back from the central counter.
HTTPS doesn't have the proper guarantees for this application.
Changing votes doesn't have the proper guarantees that people won't be buying or extorting votes. Not that it matters, because that's not really the biggest concern with evoting.
Penalties also don't matter against a malicious state actor for any abuse. As an example, Chinese spies go after the relatives in China of US tech employees to force cooperation. A state actor can stack the deck without anybody to prosecute even if caught.