Posting directly to multiple groups can’t be done for because of privacy issues and attribution of reactions. For example, if you post to a public and a private group (or a public group that’s changed to private later) - and then the item is commented on - how do you show those comments from various context? You can aggregate them because in different contexts the item is displayed to different audiences... there’s also a problem of displaying context attribution in feeds, etc. You can repost one item to different groups but that would create a new update post for each effectively and that how it’s normally done by most social networks.

Channels are more like attribution tags - they are always public and therefore don’t have contained audiences - so we can attribute one item to multiple channels.