Why we don't have Tags tab under AWS Budget Service

AWS Budgets itself does not support tagging the budget resources directly.

Alternative Solutions: AWS might expect that users who need to categorize or track different budgets can use other means such as naming conventions or external documentation to manage their budget metadata.

If tagging budgets directly in AWS Budgets is a feature you find crucial, AWS encourages feedback on their services. You can make a feature request through the AWS Management Console or through your account manager. AWS often updates and improves their services based on user feedback, so expressing your need for this feature might influence future developments.