Because it would saturate the network with queries & requeries & become slow as heck. I used a program that did that before LW. It could search the entire network (different network) but a single search could take 40-80 mins to complete. Do you really want LW to become like that?
Here's some points worth considering:
http://www.gnutellaforums.com/showpo...&postcount=126
http://www.gnutellaforums.com/open-discussion-topics/56566-differing-results.html