why did william h macy and felicity huffman have such small roles?

for such big actors their roles seemed way too short and insignificant, i don't see why macy would see the point in doing such a small role. Like I know big actors do cameos or can have small important roles where they deliver a powerful scene but neither actors scenes was that significant that could not have being carried out by an unknown actor.

Yes I know the william h macy's scene was powerful but it was still aniston that carried that entire scene and he had little input. It just seemed for big actors they never got to shop their acting chops or do important enough stuff even in their short screen time.