Mainly because this information is available via likes “already”. I’m not adamantly opposed to including e.g. Like and Announce in a representation of the conversation, but this needs to be considered more carefully as to whether it’s appropriate or not.
That point was less about “feature set” or “relevance”, and more about e.g. “should the object always be wrapped in a Create or not?” – this was discussed in an earlier topic about “Implicit Creates”: Implicit Creates – and the answer is “it depends on the convention you’re following”. A practical example would be how Streams includes the Create activity, while NodeBB includes the object. It’s not like either of these usages is more “correct” than the other. It’s just a difference in opinion.
Put another way: You can have a collection of Creates and Likes, or Notes and Likes, or just Notes, or Creates/Likes/Announces, or so on and so forth. There’s only so much you can actually constrain here; constraining too much would end up being detrimental to certain use cases.