The OP asks why they sound the same as eachother. I think that the environmental and cultural influences are key. Just like any other group of people with common interests, these people will tend to be friends, have the same tutors, go to the same Jam nights etc. The more they are listening to the same stuff, picking up chops from eachother and frequenting the same You Tube channels, the more their styles will sound similar, subconsciously or not.
As others have suggested, it takes someone with a fresh and adventurous attitude to really produce something, well, fresh and adventurous. These people will take their influences as a building block to create something original.
Only dead fish swim downstream ;-)
BTW, love that Mars Volta vid, I'll be checking out more of their stuff, thanks.