In the Twilightseries, by Stephanie Meyer, the Cullens are the "main" vampire family. Because the Cullens are trying to assimilate themselves into human society, they perform human acts, such as driving cars instead of running at the speed of sound. Since their vampire identity is largely unknown in their society, they do not want to reveal their secret. If the Cullens actually went around proclaiming their vampire status, just imagine what a different series that would be! Since they don't age, the Cullens usually have to move around every few years or so in order to ensure that their private identities remain a secret. This is also why they have to go on "hunts" every so often, and enroll the Cullen "children" in school. Hope this helped to answer your question!
The Cullens do try to assimilate themselves in the human world, not only because Carlisle, the head of the Cullens family is a compassionate and very humane vampire and he encourages the rest of the family to adopt his way of life to co-exist peacefully with the humans but also to protect their identities. The Volturi who are powerful vampires of ruling class, enforce the rules of the Vampire world, i.e, keeping the existance of vampires a secret and all the transgressors are severely punished by them. So if the Cullens want to mix up with humans, they should at least behave like humans, and do stuffs like driving cara instead of running
P.s: they dont eat vegetable or any other human food. human food taste like dust to them but if someone challenge them to eat that they can try but ofcourse that is not going to be pleasant.
On the other hand, instead of feeding on human blood, they feed of animal blood.
So that they can look normal to other people.
To make it less obvious that they are vampires. They need to keep that hidden. If people start suspecting that they can transport themselves to school so quickly without feeling tired, won't it be a little strange for a normal teenager?
So they can fit in and not look weird... DUH SMART ONES i mean my brains are mashed potatos and i figured that one out!
Because they want to be as human as possible because they are the only vampire family trying to protect the human population of Forks.
The cullens can run to school but it won't look normal and they are trying to 'fit in' with the humans. But in truth its not like they can actually fit in since they drive to school in super expensive cars. They are trying to look good with humans.
Bella asked a similar question to Edward when Rosalie drove up red car ( I think it was a jaguar) once and he told her it was all ostentatious, and that they had to keep on a good look for the public. Bella replied with something along the lines of well nice blending in with sarcasm haha.
The Cullens, who are vampires in actuality, do try to keep mixing with humans adapting the human behaviors as much as possible. They eat vegetables, solely, to show the people in the society that they are also humans. They go to schools, and do all the things which we humans do because they continue to lead the life of humans. Very similarly, so that no single doubt would occur in any human, they do drive cars though they could run with the speed of sound. All these they do to hold on the facades they wear. If it would be disclosed that what is their real identity, then they could fall in deep danger.