As said previously, the expansion adds a huge number of capturables, perhaps even more than the base game.
Don't forget that the more planets you take, the more potential entry points the AI has into your network through waves and cross planet attacks. Taking planets also has that +20 AI progress, even more if you're forced to destroy AI capturables. This is very noticeable in the expansion, as stuff like AI co-processors interfere with supply on a planet, but also have a hefty cost for their destruction (until you destroy all of them of course).
Also, if you were able to put things like fortresses and force fields on every planet, that would take away the challenge of the 'tower defence' aspect of the gameplay. Part of the challenge of expansion is that you only have a limited pool of resources with which to defend your planets. Deciding where to concentrate your defences plays an important part of the challenge in AI War, especially in the late game with huge cross planet attacks and multiple fronts to defend. Sometimes you can't defend everything, so you'll need to make an important decision as to which planets are worth the effort.