Too many lawyers. Departments of BR never had to sign binding contracts with other departments of BR.
At one time it was reported that the rail industry was employing more of them than drivers. That may still be the case.
To be honest, I don't blame the lawyers. They are a symptom, not a cause. The villains are those who designed the system in the first place, and those who were/are too timid to change it, despite knowing it is ridiculous.
Of course, it will be hugely complex, slow and expensive to change the system to something saner and more efficient - and as the web of long term contracts gets ever thicker, it gets harder to do every year.