They understand the theories and underlying principles, but the sheer amount of data makes it impossible to actually verify it.
An ELI5 comparison would be a hill of stones: you know when you throw more stones onto it, a "landslide" will occur and rearrange the hill. For a very small hill of 10 stones you may even be able to know input and output ("if I throw a stone there, the stones will be like this after the landslide"). But you cannot predict the same for a hill of 1000000 stones, even tho the "rules" are the same. You know what will happen, but you have no way to predict the outcome, or verify that everything went as expected.
The theory / math is not the problem. The scale is.
Not just the roof rack. The whole car is probably close to the max allowed weight.
The container is seemingly at around 60%, so 600L. Gas is ~25% lighter than water, so we're looking at something like 450kg.
I don't know the model, but a lot of similar or even bigger cars max out at +500kg (per specification). Most cars have less allowance.
And this is just on the roof. Not counting the container, passengers and other stuff.
Truly a marvel of engineering.