To be fair, there is also a significant lack of evidence that they haven't. It is difficult to collect data in this area because of the setup of the electoral system (both its federated nature and the lack of possible verification of individual votes). This together with the tight margins of modern American elections means potentially significant fraud could hide in the noise. In-person fraud would be hard to hide but electronic fraud is possible and there are interested, capable parties out there (e.g Russia) who shouldn't be discounted.

So to say that 'we definitely don't have an "election integrity" issue' isn't correct. There is nothing definite about it - we just have a consensus that we don't have a problem. You could say the same about every data security breach until they were discovered. I have noticed that there is a significant cultural interest in propagating the idea that our elections are sound so I tend to take the idea with a grain of salt as long as our system is as vulnerable as it is.