They have had things like this for a long time. The most famous:
http://www.netscum.com/~clapp/wireless.html
Here is the thing though, back when these devices were first being released, amplification helped, especially if it was done correctly. Now, modern routers such as the Linksys E4200 or Netgear WNDR3700 have multiple internal antenna and they use new technologies such as MIMO. Years of research have gone into making new routers significantly faster than older ones and offer further signal. If you put things in their way, it only serves to mess them up.