They have soup in the future, if only so Spock can throw it against a wall.
I think that the food on the 23rd century Enterprise is cubes, is because that's the way it comes out of their food machines. It's manufactured in a slab inside the machine and cut to uniformed sizes.
Consider present day Campbell's vegetable beef soup, little squares of beef, potato, and carrots in broth.
Kirk's "chicken sandwich" might have had perfectly square slices of bread and meat. Square slices of tomato, square pieces of lettuce.
A perfect rectangle of zesty pickle on the side.