Why are accounts divided into support / forum / main?

Question

Software design

By registering on the site, I create an account, when I need help, I need to create another account through the main one, if I want to ask something on the forum, then this is another + 1 account that is linked to the main one.
Why do this? Why can't you use 1 everywhere, cjplfnm just an additional table like user_forum_details / user_suppport_forum_Details?
Google example, 1 account for everything, anti-example cloudflare, steam, g2a. There you need to register additional accounts on forms, for g2a it’s generally crap *n * n * n**n there for each service account, and another 8 accounts can be created through social networks, but they will be like new accounts.

Answer 1

because usually these are sites on different engines and each has its own user base, and no one began to remake it into a single database

Answer 2

An example from life. On the main site, there is an option for the account to enable two-factor authentication. But there are times when the phone on which there was an application with codes has outlived its usefulness, and a new one has appeared. In this case, if one account was used everywhere, then it would be impossible for the support site to log in and write about this problem, due to the lack of code. And when splitting, the user will easily enter the support site and write a ticket.

Answer 3

Developers simply cannot tie them together, it's easier for them. Often forums use ready-made third-party engines, and few people want to fool around and spend man-hours on their correct connection.