Fear or at least apprehension is probably the basic reason. The other reason is the possible feeling of inadequacy as compared to other speakers you have heard and not having a clue about putting a speech together.
There are two parts of a speech, the message and the delivery. Some people cannot write the message, but can deliver what someone else has written for them. Some can write the message, but are uncomfortable before an audience, since writing is a private endeavor.
Some people eventually overcome the fear of an audience and learn how to write a speech that delivers the desired message.