Because they don't want to be seen like assholes, who just blindly dismiss PRs, and because they take the technical discussion about the PR in good faith.
If someone gets emotional about their PR being rejected, well... its kinda their issue.
It can be quite hard to discern this behavior from a new contributor to the project that might be a domain expert on something you are not. Possibly with the exception of reacting far too quickly & enthusiastically compared to real people that might have a life.
Edit: I see this comment getting downvoted. To be clear, I was trying to explain why someone would want to merge a PR without going through all of it, I didn't mean to call such people stupid.