Because in a big company you’ll find some things are documented in a combination of jira and confluence while another are in big figma boards and nobody knows where to look (my gf’s company has exactly this problem).
Also in a big company you want to have two teams collaborate but one uses one tool set and the other a different stack and there aren’t enough licenses for both.
And so on. It’s like having each team use its own code formatting rules or coding standard.