Why do many stealth aircraft have a similar shape and design?
First of all, the general idea is to avoid reflection of radar energy back to its source. Next, you want to direct the reflected energy into as few directions as possible in order to dazzle any observer with a momentary bright beam, surrounded by as little reflection as possible. That is the reason for the aligned edges and serrated patterns on stealth designs.

Next, there is quite some variety. How about this one (Northrop Battlefield Surveillance Aircraft eXperimental (BSAX), or Tacit Blue; source):



Or this (McDonnell-Douglas Bird of Prey stealth demonstrator; source):



Or this (BAE Taranis UCAV demonstrator; source):



If you think that all stealth airplanes look the same, maybe you are only looking at those which were designed for the same purpose?