That is because they are close to the roof and almost parallel with it. They go from a crank at the bottom of the front arms to a crank at the bottom of the rear arms. You can only see them in a good 'from above' shot taken from a footbridge or similar. Not only is this the view we normally see the model in, but much more rarely see in real life, but without this link the arms would flop about and the pan would not work, just like the real one wouldn't if they were missing...
This I don't think is an 87 cross arm, but the principle is the same so the Hornby 87 pan would need them too if it is to work:
Youtube video here:
https://www.youtube.com/watch?v=8Wvw0B3pxec