So, let's compare it to traditional paper voting. During the count there are many people counting and many people watching the count. If one or two people are corrupt and get away with adding or subtracting votes the damage would most likely be minimal.
Whereas how many cyber security engineers are there? How many people wrote this software? How many people are involved in the count. It's essentially a black box in that pretty everybody cannot look and understand the underlying mechanisms. Even if you were a security software engineer chances are you'd miss something lurking in there.