We could get into a long discussion about multinational corporations and their ruthless pursuit of the bottom line, but isn't it obvious why the park doesn't "just stay open later?" They have hourly attendance data, and they know their staffing expenses.
They don't think it's worth staying open late for day guests, but it might be worth it if they can open up another revenue stream.
I wish tickets were half price and the park was open 24/7. That would be very convenient! But that's not the world we're living in, so I might give this a shot.