Each site is for a different purpose and using different software and were created at different times so it is not like they can merge or share accounts with other Mozilla.org sites without issues. The support forum for addons.mozilla.org used to be at forums.mozilla.org and they did let you use your AMO account on the AMO support forums then. This was not without issues though. Another major Mozilla website is the general issues tracker bugzilla.mozilla.org and accounts are not shared anywhere.