WHY AWS Account is not relative each login?

When accessing AWS, there are different types of user accounts and login methods that are not directly related to each other. Let me explain the key differences:

1. Root User: This is the account owner with complete access to all AWS services and resources. When you sign in as a root user, you'll see "Root user" in the account details in the top right corner of the management console. Root users sign in using their email address.

2. IAM User: These are users created within an AWS account with specific permissions. IAM users sign in using the AWS account ID and their IAM username. Their permissions are controlled by IAM policies.

3. IAM Identity Center User (formerly AWS SSO): These users access AWS through a specific access portal URL (typically https://d-xxxxxxxxxx.awsapps.com/start) rather than the standard AWS console login page. They cannot use the regular IAM sign-in process with account ID.

If you're trying to access multiple AWS accounts in an organization:

• By default, an IAM user in a parent account doesn't have access to child accounts
• You can use cross-account roles like the OrganizationAccountAccessRole to access member accounts from the management account
• Alternatively, IAM Identity Center can be used to provide access to multiple accounts with a single login

Each login method serves different purposes and has different authentication flows, which is why they're not directly related to each other.
Sources
How as a root or IAM user do I access multiple organizations under my AWS hierarchy with one single user login | AWS re:Post
AWS login credentials not available | AWS re:Post
Logged in as Root user and having issues | AWS re:Post
Unable to Sign in to AWS Management Console with New User | AWS re:Post