Actually in the middle of reading this at the moment (great series, not sure how I avoided it for so long).
TL:DR
Jane couldn't create her own body for a couple of reasons:
- She wasn't sure what she was. Throughout the books, she isn't sure if she's machine, human, or something else entirely. With new bodies / objects being brought into being by holding a clear image in your mind (and believing / knowing that this image is true), Jane couldn't have brought the body into being - she was unsure.
- Bodies without an Ai'ue disintegrate fairly quickly. We see that Miro's shell (after he's moved into the new body) starts breaking apart, and is completely dissolved before Ender notices Ender-Val and Ender-Peter in the ship with them - a very short time. It's possible that there may not have been enough time to get back inside before the body crumbled.
- Jane cannot fathom creating or having an original idea. She says that she can follow logical paths very quickly, but cannot have original ideas on her own.
- (Low possibility): Jane is using most of her (the ansible network's) processing power just holding the images of the others in her mind. Changing that image mid-trip to include a new body (even a clone of someone else) could be too much for her to keep track of.
Recap
In book 3 (Xenocide), when Ender, Miro, Jane, and Ela went Outside, there were 3 bodies in the ship. Once they arrive outside, there are 6 - Miro, New-Miro, Ela, Ender, Ender-Peter, and Ender-Val. The 3 new bodies appeared because of the way that someone on the ship held in their mind that they actually were - Miro, because he never fully accepted his disability (and always believed himself unchanged).
Xenocide, Ch. 16 - Voyage
"I should have known," said Ender. "We should have thought. The pattern of yourself that you hold in your mind, Miro - it isn't the way you are, it's the way you were."
Ender, on the other hand, always believed that he was partly made up of both Valentine and Peter - Valentine's purity and goodness (or at least her perceived purity) and Peter's anger.
Ender's Game, Ch 1 - Third
Ender leaned his head against the wall of the corridor and cried until the bus came. I am just like Peter.
Ender's Game, Ch. 3 - Graff
"Well, Peter isn't all bad, you know. He was the best we'd seen in a long time. We asked your parents to choose a daughter next - they would have anyway - hoping that Valentine would be Peter, but milder. She was too mild. And so we requisitioned you." Graff
"To be half Peter and half Valentine." Ender
So with Ender believing himself to be half Peter and half Valentine, when he was Outside, his projection of himself came into being in the form of Ender-Peter and Ender-Valentine - neither exact copies of Peter and Valentine, but rather how Ender held each of them inside, how he believed them to be.
So why couldn't Jane just make her own body?
I think I put most of my reasoning in the TL:DR, but I can add some extra quotes as well.
- She wasn't sure what she was. Throughout the books, she isn't sure if she's machine, human, or something else entirely. With new bodies / objects being brought into being by holding a clear image in your mind (and believing / knowing that this image is true), Jane couldn't have brought the body into being - she was unsure. Such as it is, what Ender and Miro did wasn't conscious.
Children Of The Mind, Ch. 3 - "There Are Too Many Of Us"
"Use your head, Miro. Was it your decision to jump from your old body to this newer model a conscious one?"...
Miro got his point at once. Ender couldn't consciously control where his attention went. His aiúa, even though it was his deepest self, was not to be ordered about.
Bodies without an Aiúa disintegrate fairly quickly. We see that Miro's shell (after he's moved into the new body) starts breaking apart, and is completely dissolved before Ender notices Ender-Val and Ender-Peter in the ship with them - a very short time. It's possible that there may not have been enough time to get back inside before the body crumbled.
Xenocide, Ch. 16 - Voyage
"But now there are two of you," said Ela. She sounded horrified.
"No," said the new Miro. "Just me. Just the real me."
"But that one's still there," she said.
"Not for long, I think," said Miro. "That old shell is empty now."
And it was true. The old Miro slumped in his seat like a dead man.
Jane cannot fathom creating or having an original idea. She says that she can follow logical paths very quickly, but cannot have original ideas on her own.
- (Low possibility): Jane is using most of her (the ansible network's) processing power just holding the images of the others in her mind. Changing that image mid-trip to include a new body (even a clone of someone else) could be too much for her to keep track of.