From a parental point things like this irk me as DD rarely gets Housepoints as she always behaves well and gets upset when "Johnny got one just for sitting on the carpet", she's meant to read 3 books a week yet she's on Chapter books and many of her peers are still on phonic shorter ones. But I know that DDs ability to read and behave well and follow instructions will get her much further than those that need rewarding for it.
However from a professional point of view I know that they work really well for those who don't come from a Household where reading is valued or the norm. For children who are dyslexic and avoid reading at all costs. I often give credits to the teens I support for behaviour that quite frankly is expected but very challenging for them to meet.
Generally brighter children (don't assume all the older ones are on longer books) come from homes where reading for pleasure is encouraged and a part of life, these rewards like Attendance ones are not aimed at your child. They are aimed at the children whose parents don't value reading/school as important as other areas. For the child who reading a 20 page book takes as long as your child reads a small chapter book.
Who wants them to win your children or you? If they want to win then they simply need to read more. They presumably have the option of choosing shorter stories at their reading ability Roald Dahl books are fairly short, David Walliams one might take 3 nights if they are pretty competent. Perhaps they need to work on their reading speed so this could encourage them to do so. if they want to read, make sure they take a book wherever they go, car, bus, train. Don't watch TV and read a book instead.
Or, they could just read for pleasure and fit it in around other activities that make for a well rounded individual and not worry about Housepoints.