The vast majority of review actions don't require knowledge of the subject material discussed in the post. Only a handful of such posts require domain knowledge.
Restricting users to reviewing content that they have actively contributed to would dramatically limit the number of valid and appropriate reviews that they can make, while removing only a small number of posts that they shouldn't be reviewing.
As such, adding this restriction would cause more harm than it would help.
An entirely different "feature" is to simply apply a preference to tags that the reviewer is active in, without actually requiring them to be active in the related tags to review the post. Note that that feature has existed since the review system was revamped last year. You're much more likely to be shown posts in tags your active in, if there are any. When you're shown posts from other tags it's generally because there are no posts from tags you are active in. (Note that this is dependent on the queue, some I think don't have this feature, some do.)