Why did manufacturers tend to change the body style each year in the classic era?

Just as an example with Chevrolet Biscayne there are 18 different body styles in total with each new generation or generational facelift, ranging from 1958 – 1975

It just seems like manufacturers are inheriting the same body designs now, with no changes to the design and just slapping on a new year on it, until a redesign eventually takes place