The invasion system is as it is, because it is by far the simplest way to code a generic solution that "covers" all eventualities.

The commanders level is the input for a generator and feeds into a simple reward model and then applies the underlying first map of the zone where the map generator placed the invasion counter.

The more regular approach of a tiered, tailored set spread across fixed level intervals with a set of rewards balanced to fit into the larger game would take way more time and resources that the small team behind AB has at least in the current situation where it, at least to me, looks like they are stuck in a marathon game of catching up.

I'd go as far as to claim that the invasion state also reflects that there is simply no option but to throw basic versions to the gamers, in order to provide content and also see what floats and tinker accordingly on it.

On the up side it means they really do try. On the down side it means that while the walls are those of a palace, you are going to be sleeping in a tent for a while, at least while the rest is being built.