Answer
Aircraft may approach from another direction, be masked by buildings or wind, or become visible only when already close. The pilot should use official airspace information and keep the operation within any authorization conditions.
Explanation
Lookout is essential, but it is not a substitute for preflight airspace planning. Airport traffic may be intermittent, fast, or hidden until late. Official airspace data gives the pilot information that eyesight and hearing at one moment cannot provide.