There's ratings for tv series as a whole & individual episodes, but why not seasons as a whole? I think you can get a better idea of what people thought of a season with it, instead of looking at every episode individually.
One of the primary reasons that they don't do this is that there is no actual Season level in the database. The only data in the database that has anything to do with Seasons is the integer portion of the Episode Number.
It is relatively easy to show data by Season just by Selecting or sorting on the Season portion of the Episode number and combining data as necessary.
However, in order to allow Users to rate Seasons, a Season level would need to be created, including processes to create and maintain each season and maintain links with the Series and Episode data. This is a lot of work for a very small return.
IMDb might agree to display Season ratings that are derived from the Episode ratings, as this does not require changing the underlying database, but I don't see them allowing users to add Season ratings.
I really wish IMDb would add the ability to rate individual seasons, too....some seasons are great, and some just don't stack up. Additionally, some shows feature seasons each with self-contained stories (American Horror Story as a recent example). It would be nice to rate and see data as a whole for any particular season in addition to the show overall and individual episodes.