Deliberate mistake: If you pay extremely close attention to the scene of all of the Decepticons responding to the small Transformer's message regarding the location of the Allspark, notice that the shot of the police car and the fake helicopter pilot are duplicate shots used earlier in the film.
Deliberate mistake: At the corner of the roof, Sam clutches the statue as he protects the All Spark from Megatron, who then destroys the statue causing Sam to fall. In the overhead shot of Sam falling, as the chopper flies by, we see the All Spark leave Sam's grip and actually fall behind him. Yet, in the next close-up the All Spark is safely held in Sam's arm again. (02:06:50)
Chosen answer: There is no reason to think they can't. They simply choose not to. They may feel it's too deceitful.
Actually in this movie (when joining up with bumblebee and the others after they pick up the All Spark) and Revenge of the Fallen, Ironhide uses holographic humans in his cargo bed. Also, Arcee and the twins use holographic drivers at the beginning scene in Revenge of the Fallen.