It may also be something that just wasn't commonly used or thought of.
Even in our world, lets use the concept of hauling bricks. Some people will just carry a few of them and then turn off their brain and just keep doing that. Then someone comes by to point out something, lets say a wife, stating that the neighbor has a wheelbarrow that can be asked to be borrowed. Or someone else, a kid, might get the idea of using some cardboard to slide the bricks instead of carrying.
I do have to assume that the people in this manga world do have the concept of creating blocks which would be easier to make buildings. However soldiers who aren't in the building industry might not have the concept if they're not doing the moves of breaking rock and creating blocks constantly.
Also, if we consider physically moving blocks (even by slime)... why blocks? Why not spheres to roll them out of the way and block them later if so wished.
Worlds progress with the "Why not try it this way" as well. There's plenty of instances in our world where it can happen.
Take the assembly line concept for example. It existed, but the concept was then applied to cars.
And then there is the situation Leo mentioned. Any of my programming is going to be very step by step, non-optimized / non-efficient because I'm at the create variable, do something with variable, do something else with variable stage, where I *know* others can create something more efficient and combined steps.
...Now I recall using a lot of nested if/then statements and then learning about case statements, and even with that, I'd gravitate towards if then until I realized it would be more efficient to use case.