There is a rumour that Ive is introverted and had a fear stage. However the truth is that Apple does not want to break the "general formula" kept for decades
The launching of the Apple products are extremely precise and careful that CEO Steve Jobs spent years perfecting his presentation skills. During more than 20 Apple presentations, the only thing that changes is the product name. The role of each of the show was clearly divided, and they will do the best their mission

Jony Ive - Apple's Design Director.
As usual, Apple usually chosen Tuesday to launch the product. At that time, the CEO will step on the stage and thank journalists attended the event, then he would say, "we have some great products want to show you today" .
First, the CEO will give the figures to prove his company is still growing very well. For example, if they introduce the new iPhone, they will talk about iOS earlier. Before launching new products, the CEO will give the figures show that how the older products have sold well”
And then they announced new products with a short beautiful videos. Regularly Apple just introduces some outstanding available on the device, the others will be found out by users and press later. Before Steve Jobs died, many people take on this role such as Philip W. Schiller - Vice President of Marketing Apple.

Then, Jony Ive leads stories in a long clip introducing the production. He wore a T-shirt G-Star Raw. Jony narratored when the beautiful video about the product is presented. Jony Ive will be the first and the last of the video. He did very well his role in the video, and Apple does not want to change his role.
Some other reasons why Jony Ive does not present directly, he does not want to take much time for presentation training. He pays attention for the video. Some people feel that Ive is introverted and had a fear stage