The world was very small. It was just one city, and there was no reference to anything outside of it.
The world also felt empty, despite being ostensibly populated by people. We almost never go to see even a hint of an emotional state of anyone outside the main cast, and in many scenes and shots where there should have been people, there simply weren't any. Of the people we do see, they all feel flat and one-dimensional, with the singular exceptions of Rika and Akane*.
There is also a very strong repetitive pattern to the events of the plot. Akane makes a new kaiju, Yuta and the gang scramble to the junk store, Gridman appears and saves the day. There are variation, but the basic structure remains exactly the same, at least until the final few episodes.
Note how these are all Akane's own frustrations.
* There's also the glasses guy, but his character only gets "unfolded" when the world around him breaks down towards the end.