In the past it was always included in the introductory letter that VM sent out, I don't know if that is still the case. As to why it isn't sent by email, that is either for security reasons as email is not secure and they want to make sure only the person resident at the address gets it or poor planning on the part of VM. Take your pick 😏
I don't remember it being crucial to "setting up my services" as the engineer who installed my kit setup my services, but I did register a long time ago so things might be different now.
My setup: V6 TV box, M350 Fibre broadband with Hub 3 in modem mode connected to a Netgear R7000 router. Telewest/VM user since 2001.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.