As I see it, the problem is you don't know how those far hosts which are maybe 7 hops away from you, are connected to the rest of the network. They might not know other hosts than you do and they will forward your request to the hosts within their reach.

If you wanted to be sure to reach any new hosts, you would have to try more than one host and your horizon and it could as well be that your search-request reaches the same part of the network multiple times, causing additional traffic each time. If this was common place, the amount of search-requests could easily multiply, without necessarily improving the result of the searches to the same extend. This could be very ineffective.

I think supernodes are currently the best solution to increase your network horizon. But I'm no expert. Maybe one of the developers knows it better than me.