Maybe you live in an area that aint as speed camera happy as other places.
I know round here you just cant speed they are everywhere.!
Considering it's quite a serious offence (And people do get done), then it's a decision not to be taken lightly.
E.g.:
http://www.timesonline.co.uk/tol/news/uk/crime/article2017307.ece
Seriously not worth it.
Humberside police 'let me off' my last potential three points,
I had to pay £70 and attend a speed seminar, which I thought was better than 3 points and £60.
Returning to Humberside was not very nice though.