Getting the dinosaurs out of the zoo wasn't part of the initial plan. Sobek had wanted to be captured only to arrive undisturbed in a city, gain fame points and unlock all the boxes of the [Weapon system]. However, he had decided to take another advantage of that affair, taking away some dinosaurs to increase the initial ranks of his army.
If Sobek had had to recruit new dinosaurs on his own it would have taken a long time: most dinosaurs were very fast and would have run away the instant they saw him. But if he had had a good initial number of subordinates on his side, then they could have done the work for him, covering much more ground and drastically reducing recruitment times.
Thanks to the zoo, therefore, he had obtained dozens of dinosaurs in one fell swoop that would have allowed him to quickly recruit many individuals.