Why Roles in League of Legends so well defined

I started play LOL recently but I have 4500 hours in dota 2 . I find LOL to be a fun game but why roles are so defined in this game ? I feel like the whole game is so static and less dynamic than what I used in dota . many of the champ you have here remind me of the one we have in dota 2 so its kinda easy to get into . some of them are combo of 2 heroes in dota 2 so get into the game it easy for me. I understand the mech of the game. but the fact its so static and you cannot easily teleport to any part of the map make the game very weird for me. I feel like I am stack in one part of the map for so long without making any rotation. But back to the roles. why roles cannot be more free in this game ? also the items don't have much of diversity. seems on every Carry I build same items almost same order . no carry feels different in sense of what I build on them. its all Crit , damage , attack speed. hmm . tanks seems same. why there arenrt any game changer items ? most items are very blend