Because, to repeat the points already made once - the splash damage models an explosion (however imperfectly),
Right, but it's modelling it even more poorly, in my opinion, than what Great Weapon Fighting is trying to model. It makes no sense at all that alchemist fire can burn through +5 Plate mail every single time, or that it will always hit even the most agile person, or that it splashes in exactly equal portions to every 5' square within 5' of the square it lands in, as opposed to continuing to mostly travel in the direction it was thrown due to momentum. These are all modelling issues which simply don't make sense - less sense even than the concept that you can always make a glancing blow on even the most agile person next to you with a really big weapon.
the splash damage in non-discriminatory, affecting friends and foes alike, and the attack burns off resources. The great weapon fighter is not exploding, never hurts his friends and burns off no resources.
These last issues are all ones of balance and fairness of the mechanics. But, as we've already explained, this option for the fighter is in general mechanically much weaker than the other options, having even less impact as the game goes up in levels. So, in the context of where this option is at (relative to the others one could choose), it seems balanced and fair, perhaps even unusually weak.
Which is why I have troubling understanding the severity of the objections to this option, though I acknowledge the severity is real for a fair number of people. In terms of believability, the ability seems more believable than the already-accepted splash weapon mechanics, which model things even poorer than this option models things. And in terms of balance and fairness, this option is just as balanced and fair, or even more balanced and fair, than the splash weapon that is already-accepted. So why is the reaction so severe for this, but not for splash weapons?