People keep using "meta" like it's a bad thing. It is literally the most optimal strategy that we know of based on the game's current state (the definition of meta-game).
The reason we have an ADC is simple: a standard build ADC has the most damage potential at full items, regardless of level. We put a support with said ADC because, as I said, leveling up doesn't do as much as getting items. We put mages and casters in the middle lane alone because they scale really well with both levels AND gold. Putting them in the middle of the lane allows them to use their earlier power to dominate the entire map, rather than only their lane. The jungler is meant to give map presence throughout and maintain objective control. These champions have generally good scaling, but have a kit that benefits other players more (CC and the like). Now, our top laner is just another body for most of the game. In our current meta, being tanky and with plenty of CC is favorable, so they can have the mid-game teamfighting presence that teams need.
The meta exists for a reason -- because it is the most effective way to win. Having a good mix of both magic damage and physical damage, a presence throughout the stages of the game, and objective control is a base strategy that has been proven.
HOWEVER!!!! This isn't the be-all-end-all strategy; far from it! I'm a huge fan of playing against the meta, mostly because I don't play to win; I play to have fun. I recently picked up AD Malzahar Jungle -- and I love it! Did you know that Malzahar's voidlings' base attack damage at max rank are the highest in the game at level 18? They're only beaten by Baron Nashor. Pretty crazy, man.