I thought so too. The biggest reason, I think, is what
Zigūr said - there's only so much you can do with visual manipulation, and the actors already had to wear fatsuits.
However, there's also a risk of making dwarves look too stocky. Last summer at the Return of the Ring, Tolkien artist Jenny Dolfen told an anecdote about somebody suggesting that the width of a dwarf's shoulders should be five times their head (while normally the width is 2-3 times the head), effectively resulting in pictures that look like this:

(Yes I really just made it for you, I'm that bored.)