Because instead, they chose to go with the most chaotic, haphazard, and frustrating system I've ever seen.
You see, the very concept of "tank and spank", or rather, predictable mob behavior allows for planning and execution in many more ways than one. Current GW2 system allows for what, graveyard zerging most of the time?
I'd rather say, lack of mechanics.
The main principles of GW2 PvE combat can pretty much be described by "don't stand in fire" and "run away when anything looks in your general direction". Because both aggro and most AoE abilities are quite random, there is hardly any way to predict who gets to be "it" the next second, so your best/safest bet is to have as much space between you and the target at all times, and for melee characters it's just not an option, even traited for Toughness/Vitality with "CC" utilities. I'd say it isn't "innovative", it's just poorly done and/or poorly balanced.
Heck, even Tera, being lackluster in so many areas, managed a much better combat system which included active rolling, attack blocking, positioning and whatnot. In GW2, as it stands now, you have a choice between kiting or graveyard runs (sometimes a mix of both), with survival-oriented builds being favorable for most situations. I sure hope it's not "working as intended"...
Edited by Xenobius, 16 September 2012 - 11:15 PM.