Hm, it's computationally expensive thing to do and the crawler must be feeded and "crawl" 26/7... I mean either is run by a community running many many nodes or an individual with huge resources, otherwise isn't worth it.
I'm not sure why a federated search engine much be running 26/7 ... what's so prohibitive about federating it?
[1] https://news.ycombinator.com/item?id=3288586