There may be reasons, but it's a negative for guest satisfaction.  The uninformed get to the park 30 minutes "early" only to find that the park is already open and there's already a wait for major attractions because Disney purposefully obfuscates the opening times. 
Or alternatively, you're a planner like me and so many others on this board, and you can't really know when a park will necessarily open its parking lot, its gates, or its attractions.  It seems like there could be consistent rules.  For example, parking opens at T-60, turnstiles open at T-30, rope drop/attractions open at T-0.