One of the problems we have in school is that children are not able to make links between different topics within the same subject, let alone linking different subject together.
When the link is explained to them, they are surprised, and comment that they didn't think it was allowed to link the topics together.
Part of the reason for this is the way the education system is structured in to different subject, and within different subject, it is broken down in to different unit. Much of it is purely for administrative purposed, but it does out students a disservice.
Cross curricular activities are a way to redress this situation, but there is a reluctance from teachers to take them up due to their own subject area pressures.