»Why can't they "Jack-in" within Zion?«

This has probably been asked before, but why not jack-in in the safety of Zions' walls? Seems the safer way to connect to the matrix without Sentinel bother?

any ideas?

because then zion would be detected and they wanted to stop that...later that becomes irrelevant

Niobe: "We're gonna do what Commander Lock ordered us to do. We'll evacuate broadcast level and return to Zion."

They need to be overground to hack into the matrix

exactly, Zion is to deep underground to be able to broadcast, the only place were they can do that, is above ground, just like a radio, it won't work underground, can't pick up the signals. jacking in works similar to that.

No they actually have to tap the lines that carry the matrix signal. Some of these run above ground and most run below ground. The ships (though its not shown) have a way to tap into these lines and run there program from there. Hence why you never see an excaping ship with people jacked into the matrix otherwise it would be fairly easy to hack a line just by flying around the sewers and staying moblie so that the senteniels couldn't track thier position, And why they need specific hard line or Land line phone to get out. Certain land line phone are directly connected to the Matrix lines that run underground.