I don't think much is too contaminated when it arrives at Ford MRF, I've seen West Sussex CC reports that say
around 80k tonnes are collected at the kerbside, and total sold on from the MRF is listed as 76596.46 tonnes.
I know the Ford MRF is one of the most advanced in the country, and they separate plastics on site before selling on, it was featured on BBC1's science magazine show 'Bang Goes the Theory' about 10 years ago, sadly that's not on iplayer now, but there's an article about it
HERE.
So, that's probably one reason, but another is explained at 2.50 of this short video, basically West Sussex CC thought it was important to get long term markets for what they sell, at reasonable rates, unlike some council that cherry pick for high prices that are only short-term.