Why is it that the ships in the Star Wars prequels look more modern than those in the originals?

The first three Star Wars movies were created beginning sometime in 1975 -1976 or earlier and shown around 1977 with some two-year intervals. This means digital and computer animation in those times were cruder than the recent ones which were done in the 2000s. Generally, if the recent Star Wars prequels copied exactly the spaceships of the 70s, they would not look good as other scifi movies would have then shown better looking spaceships. Forget about consistency as this is a smaller matter compared with the new art and the excitement it brings. Just imagine how "Avatar" would have looked like if it was produced in the 1970s.