Or else because the car is not yet ready, it needs to be better tested and perfected in the laboratory, especially now that time is tight and there are only a few days left for the official races.
It is also possible that they want to prevent any accidents, damage or extra costs that could result in major problems that may not be resolved until the start of the championship.
Furthermore, I believe that even pilots feel more "comfortable" in training with a car they already know.
If you had some Toyota corolla out there with wings, it would still be the same, because the idea is to find what is better, not to test the current car but to test the setup. I am sure there are million other things that they look for, but that part of it is definitely important, the fraction of speed you gain from even one millimetre difference in the setup could gain you tens of seconds in a race.