Why can't I add existing email to aliases

Hello, first it's a pleasure to be able to help you today.

My name is welcome to the community.

Hello LunarDemon3326, Microsoft created the system this way, but I believe this is so that there can be no conflicts with other possible existing email accounts, so you'll always have to create an alias from scratch so you can add to your account.

Add or remove an email alias in Outlook.com

https://support.microsoft.com/en-us/office/add-...

If the email address you're using for both accounts are from a non Microsoft domain. You're correct, that's possible

But you can review that's stated in the article notes:

"You can't add an email address that's associated with a work or school account as an alias.

Existing Hotmail, Live, Outlook.com, and MSN addresses can't be added as aliases, and you can't add an alias that is already associated with another Microsoft account."

"You can add your email to your existing mom's email'

From my understanding all aliases will have the same password, is this true?

BTW does it matter that my email is with gmail?

Hi and thanks for reaching Microsoft Xbox gamer's community. My name is Josef.
I'm an Independent Advisor and a Microsoft Xbox user like you.

Hope you're doing great today despite this CHEROKE779

If the email address that you're trying to link belongs to a microsoft domain, as stated in the aliases FAQ.
You can't use that email address. Instead you need to create a new outlook.xxx address or add an existing email from another provider (Google, Apple, etc)
https://support.microsoft.com/en-us/office/add-...

"Notes:

You can't add an email address that's associated with a work or school account as an alias.

An alias or email address can only contain letters, numbers, dot (.), underscore (_) or dash/hyphen (-). Spaces and other special characters aren't allowed.

Existing Hotmail, Live, Outlook.com, and MSN addresses can't be added as aliases, and you can't add an alias that is already associated with another Microsoft account."

go here ->
account.live.com/summarypage.aspx click on aliases, verify yourself, add alias, verify it if you use an existing email, make as primary, remove old email.