Because we do, that's why!!!!All kidding aside, I've noticed that the "tone" is more player dependant than the equipment a lot of times. I mean if I play a Les Paul with active pickups like Zakk Wylde does I can get a sound that approxiated his through my Peavey amp, even though he plays through a JCM800 Marshall. But I can't if I'm using regular pickups. So the equipment has something to do with it. But on the other hand I've seen really good players pick a POS guitar and run through their equipment and they still sound just like they always do. So a lot of it has to do with personal technique I think.
Why do a lot of people like the sound of a LesP through a Marshall? Because a large majority of music has been recorded/played live through this equipment so that's what we equate with good sound. Now I think a strat through a Marshall sounds weak, but completely awesome through a Fender Twin. Whereas a LesP through a Twin doesn't ring right. Why, because most of the blues players I've ever equated with being good play the strat/twin combo. Not all, but most (exceptions like Albert King, Lonnie Mack, etc..)
And the get all---most active guitar players I know try to listen to as much live music as they can cuz that's where the action is!
Was that meaningless enough for ya'll.