I think because capitalist employment is inherently adversarial. If employers (and managers) reveal the time budget, employees may take advantage and reduce output to expand to fill the deadline. Tight schedules squeeze employees, so hiding the real time constraint allows management to exert pressure by adjusting the deadline. Employees that realize the bluff and ignore fake schedule pressure can be identified, marginalized, and eliminated.

Avoiding this degrading game is half the reason I preferred contracting.