Sure, just like how you can just steal physical absentee ballots out of mailboxes and mail them in.
There's no such thing as perfect security. The problem of secure elections is how secure is secure enough?
How many keys/ballots can you feasibly steal before getting caught? Is that number significant enough to change an election? Historically, it hasn't been.
Or just pay you. A huge portion of the populace would vote "the correct way" for a remarkably tiny kickback. We saw rampant vote buying during the gilded age.