Units probably have to much health and heal to quickly. Because splash damage seem like a small factor here. That should be the thing to discourage fighting with a blob, but it really isn't. Mind storm is really the only thing that is tried to be avoided, but you need several of them to make any kind of difference. (I speak more from what I seen than played myself on this one)
It's more complex than that. Splash damage is very effective. If there is splash damage present you don't fight it with a blob, you split your army, or I think scatter is the better term to use, or disengage for a second. The problem is that in SC2 army movement is too easy, high ground, choke points, static defense aren't much of an advantage because it is ridiculously easy to swarm these defences. Unless it's tanks, the dude spends his spells and you engage again. As cowboy wrote, in BW trying to get a few dragoons to shoot the fucking tank on a highground was an exercise in frustration, or getting your 2 groups of marines to run through chokepoint took 2 minutes. In SC2 you just box whatever shit, click past the chokepoint, some units get killed in the process but whatever, and attack. The concept of defending against superior attacking force is absent in SC2. If you want to defend something, you need to have similar force as your opponent. The exceptions are very few, the only units currently in the game providing area control are tanks I think(and they are shit vs toss), also sentries and infestors make defensive scenarios work due to their dumb spells. There is also other side of the coin, with unlimited selection and better pathfinding, reinforcing defesive poistions is also easier. Thats why games of SC2 very often devolve into sitting on 3 bases and doing drops or air harrasment while dancing with the army around the map until you get deathball, while in BW you get engagements and skirmishes all over the map.