Not to mention AoE attacks making splits important, as well as melee units still requiring careful setup of engagements for success. SC2 is extremely micro intensive, especially for how fast it is (in fact I dislike how heavily weighted it is towards success), and it feels a lot less like you're spending your APM fighting the game and your own units and more like you're helping your own units against your opponent's units.
SC:BW still had AoE attacks, arguably more important ones too.
Nuke and Psi Storm exist in both games, but Plague, Irradiate, Ensnare, Stasis Fields are all AoE effects... as well as AoE damage sources (Tanks, Archons, Valkyries, Devourers, and kinda-sorta Mutalisks).
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Lets put it this way: if you really wanted to, you could just have 12-units per group in SC2. But no one would do that, the APM-advantage of having a big group (and manually splitting in the rare cases where a split is needed) is far superior than using 9 groups to split up 108 units.
And sure, stronger players probably split their death-balls into smaller groupings of maybe 30 units or so (you don't want to just A-move all over the place, a degree of flexibility is useful), and only occassionally merges everyone together into a deathball. (Fast and loose is good: you don't want to dedicate an entire army to a battle, especially if they might lose).
But no one is making groups of ~12 in SC2. Its too small.
