Except that it is stated clearly in the novels that halflings can’t wield arcane magic, which is why Rajaat used humans as his champions.
Now, you can argue that being shadow people means they are no longer halflings and that the nature of their transformation means they can now do cast spells, but one thing that’s interesting to note is that in the CS they partially emerge from the black as halflings:
“Khidar?” Tithian gasped. “I thought you were a giant!”
“Of course not, you imbecile,” Sacha chided. “The shadow people are descended from the last of Rajaat’s halfling servants.”
“Shadows play strange tricks with size, do they not?" Khidar added, grinning. He now had a fully featured face, with short-cropped hair, blue eyes, an upturned nose, and bright white teeth. "Your ignorance is understandable. There weren’t many of our people. Most halflings of the Green Age wanted nothing to do with the Cleansing Wars.”
Tithian ran his eyes over the devastated park, not at all interested in the history of the shadow people. “I don’t suppose you can tell me where to find Rajaat.”
Khidar pointed a black finger toward the edge of the burning thicket. Although the halfling’s head was now completely solid, the rest of his body remained a mere shadow. “Rajaat has told me you must look for him in the heart of Ur Draxa,” Khidar said.
and later:
“Don’t worry about a disguise,” said Khidar. “I’ll make certain the Draxans are too busy to concern themselves with you. Besides, until you destroy Rajaat’s prison completely, my people can emerge from the Black only partially. (this support RaFoaDK that they are tied to Rajaat’s prison in some way) With us wandering through the city, you’ll be only one of many strange things loose in the streets.”
Soon, they came upon three strange beings leading a dozen slaves after a portly templar. The creatures resembled the ancient halflings of the Blue Age, save that they were part shadow and part person. The leader had a material head and a shadowy body, while another had solid limbs but nothing else. The third was split down the center, half silhouette and half physical.
When the leader of the half-shadows saw Rikus and Sadira, he called out in the strange language of the city. Though she did not understand the words, the sorceress recognized the voice speaking them.
"Khidar!
One curious thing is the way Sadira defeats them, which shed some light on their nature (pun very much intended):
Then she remembered Rikus’s description of his fight with Umbra. Even with the Scourge, the mul had been unable to defeat the shadow giant until he dropped his torch. The weakness of Khidar’s people, she realized, was that without light there could not be shadow.
Sadira turned her palm toward the ground, preparing to cast a darkness spell. She felt the energy flowing up her arm-then the familiar tingle abruptly vanished when it reached the black stain on her shoulder. The half-shadow gripping her by the collar screamed in pain, then suddenly released his grip and fell away.
He looked as though a bolt of lightning had blasted away part of his body, with wisps of black smoke streaming off the empty place where there had once been the silhouette of a shoulder.
At first, Sadira did not understand, but then she realized what had happened. Shadow people had no life-forces of their own; they existed only as silhouettes marking the absence of energy-usually in the form of light. So direct contact with a mystic power-one of the most potent forms of energy-annihilated them.
All in all I think this is a case where the cruelty of the SMs came back to bite them, they used Rajaat’s halfling servants as part of the spells to imprison him, and probably thought they are gonna be helplessly imprisoned in the Black and die off, and didn’t even consider they would survive and adapt (most likely with Rajaat’s help).
, and halflings simply “can’t” (i e., don’t) use (arcane) magic b/c he didn’t teach any of them it - he’s demonstratively insane, so its not a far reach to imagine that Rajaat couldn’t handle the idea of teaching the “perfect” halflings arcane magic if he was going to try and reboot the world to the Blue Age; halflings didn’t have sorcery in the Blue Age, so halfling wizards would break that perfection.