Probably just the way their system is set up. They could change the system, add the features needed to bypass that aspect but that will get bogged down in their (probably already understaffed) IT department backlog. So just put it up in the current system and create a couple of minutes faff for us rather than it never getting released.
Whenever a website requires a boatload of personal info I just enter blank values like N/A or 0 into all fields. No-one ever complained.
Though it is annoying that I need to register at all.