I believe people (myself included) are simply not aware that them reviewing pull requests would do any good. By definition, pull requests must be reviewed and can only be merged by these which have the authority to do so. So it seems superfluous when somebody like me adds a "looks good to me" comment under a PR. It has to be checked by the merging person anyway.
It's still helpful though. Even simply chiming in to say that a particular feature may be useful without actually reviewing a PR is helpful as it identifies the PRs that reviewers should be focusing on (i.e. stuff that people actually want and will use).
It works in reverse as well. New features mean new code complexity that needs to be maintained over time. Chiming in to say that a feature makes no sense, doesn't cover a edge cases, or that you simply would never use it helps decide if a given change will be worth it in the long run.
