YABalittlebitU. I suspect like hocuspontas that it's new people (yes, I made this mistake when I first started using mumsnet!) who are looking for specific topics, often because they've been told to by old hands. They then post on ancient threads, which bumps them to the top.
The dates of threads are flipping tiny, and even for non-novices it can take a while to notice that the OP date was 2002 or whatever.
IMHO the worst aspect of Mumsnet is the impossibility of organising threads by date of OP, rather than latest thread. That would sort everything in a jiffy.