While fruit and veg contain lots of natural vitamins and minerals that our body needs, a carrot shape or an apple shape is quite an inefficient way to package the things you need. That’s why most space food is all pureed and mushy, because it takes up less space, and they can add extra nutrients if necessary.
But fruit and veg that can normally be stored at room temperature is eaten on space flights quite regularly. But c’mon, if you were in space with noone to check up on you, would you really want to eat fruit and veg all the time?!
The international space station has deliveries of fresh fruit and veg every so often. Like Leila says, real food containing important vitamins is inefficient. You can achieve the same result with a multivitamin pill.
It actually comes down more importantly to psychology – human beings like eaten varied diets with different colours and textures. If you just eat horrible vitamin enhanced sludge all the time you would get pretty morose, which given all the other stresses of working in space, is not a good thing to happen.
NASA actually spend huge amounts of money just researching astronaut food. They’re paying a load of people heaps of money to live in Hawaii (!!) doing nothing (!!) and eating possible astronaut food for a mission to Mars, to investigate the long term effects of doing so.
They develop new space food all the time, but I don’t think it ever really matches up to a home cooked meal!