Because they havent added it :D Dynamic membership for Azure AD groups is a feature that was launched on Azure AD side of things, it is tied to AAD Premium licensing and is still only managed via AAD tools (the portal, AAD PowerShell or the Graph API). I do agree that it's a good thing to have it in the M365 AC as well, but this is hardly the only feature that is missing...
You can leave feedback/feature request within the portal or on UserVoice.
Creating M365 Groups
H Raja
326
Reputation points
I'm creating a Microsoft 365 Group (dynamic user type) and I'm wondering, why is it possible to add an Dynamic Query via Azure AD Portal. But the same option is not avaible via the Microsoft 365 admin center, when I create a Microsoft 365 Group? why is the option not available via 365 admin center or these different groups? Example screen shots below: 
Microsoft Security | Microsoft Entra | Microsoft Entra ID
Microsoft Security | Microsoft Entra | Microsoft Entra ID
A cloud-based identity and access management service for securing user authentication and resource access
Answer accepted by question author
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Vasil Michev 123.6K Reputation points MVP Volunteer Moderator2021-02-23T18:44:01.973+00:00
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H Raja 326 Reputation points
2021-02-23T22:49:32.13+00:00 Thanks I'll feedback/feature request. Just confuses people if can be done one way, but not other way. Even though it's the same item! Appreciate the quick response.