Why is there so much precasting?

Sometimes precasting to some extent is a necessity. It would be a brave director who was planning to do say "Hamlet" and just hopes that someone talented enough turns up on the day.I like to know that I can cover base, but do not precast as such (with some exceptions). For example, casting Miss Bosnia or Five Women Wearing the Same Dress, I like to know that I have Actor x and y and z showing up, because they are versatile and I can cast them in several parts in that age range if necessary.One director I know recently held auditions, but not for the parts that were precast - because the play was in many respects a sequel, and some actors were reprising roles. Another actor was precast for other reasons, and the part was not on the audition list. A solution of sorts.I have not held auditions for one act plays I have directed in the past few years. The reasons were personal. Last year I was heavily pregnant during the rehearsal period and I wanted the rehearsals to be stress free. Consequently I cast actors that I knew were reliable, would be willing to rehearse in a hospital room if necessary, and who were not prone to hysterics or tantrums. I must also admit that when I read the play, I saw it as a vehicle for these actors.I agree though. It is wrong to audition for a part a director KNOWS is going to an individual. As an actor I have seen it happen and was actually told by a director minutes after an audition, that he had already promised the part to someone else. Annoyed me for wasting my time. (Actually the actress then became pregnant and I got the role any way...but that is another story.)My only advice is to ask when you first enquire about auditions. It might encourage the precasters to "fess up".Kimberley