I agree, most guests have very few reviews. But, I think the point here is, can a host set conditions such as a required number of reviews for a guest before accepting them? It would appear not, that's the way Airbnb works.
You could ask Airbnb to add this as a feature, but I doubt they will.
One option might be to reply to requests without accepting or declining. You could send a text reply along the lines of 'One of my rules is...' and add that if they meet the condition, you'll happily pre-approve them. Then wait for the response. I suspect most potential guests will, at this point, go away. And, you won't be penalised for declining requests.