There is a meme out there in the wild (or at least, I think there is a meme) that many reviewers would simply approve PRs blindly leaving a LGTM (looks good to me) comment when their internal monologue is closer to "...I have no idea what's going on here and I'm too afraid to ask at this point". I suppose it could also be sheer laziness of not wanting to parse many lines of new code, especially if you're in a situation where you don't have a 100% tight understanding of the codebase (as I have been in many a times).