Building House in Vietnam.

There are things in Vietnamese house building that I don't understand.
Maybe someone can explain it to me.

1)
For a boundary wall (only for work, not for material) you pay an amount per meter (not m2).
The height is calculated from the edge of the road upwards.
So if the land is 1 m below the edge of the road and I want a 3 m high wall, that's 4 m. If the land is on the same level as the road and I want a 2 m high wall, that's 2 m. But the price is the same because it is calculated by meter, even if it is twice as much work.

2)
In Vietnam 8x8x18cm small bricks are usually used for construction. If I want a 20 cm thick wall, these bricks are laid twice.
In my country, bricks four times the size are used. I estimate that you could build a wall with these bricks in half the time with half the work (that would be more lucrative for the construction company since the wall is calculated by meters).
And with large bricks, less than half the cement would be needed.
Cement here in Vietnam is a lot more expensive than bricks.
So I pay a lot more for the material just because here in Vietnam they work with small bricks.

3)
The construction company wants to mix the cement or concrete himself.
However, concrete and cement can be delivered precast-mixed, which would be less work for the construction company and he could save money, since he charge by meter (or in m2 for a house). The construction company says that the cement or concrete delivered precast-moxed is of not as good  quality as self-mixed cement or concrete. But I think if a high-rise is built here in Vietnam, the concrete or cement is not self-mixed either.


They could build a house or a boundary wall for the same money with less work and in a shorter time if they would use precast-mixed concrete and cement and larger bricks.
So what could be the reason that construction company in Vietnam voluntarily take on more work?