I think that the question is written from the perspective that the conference is somehow an independent entity from the sponsoring organization. But that feels somewhat ahistorical to me. To have a concrete example, consider a society I am a member of: the Association for Symbolic Logic, founded in 1936. According to their web site,
The Association for Symbolic Logic is an international organization supporting research and critical studies in logic. Its primary function is to provide an effective forum for the presentation, publication, and critical discussion of scholarly work in this area of inquiry.
Of course there are societies much older, and much younger, but presumably each was founded by a group of individuals who agreed that a common organization would help their goals in some way.
Common activities of societies like the ASL are:
- Publishing journals and monographs
- Organizing periodic meetings
- Awarding prizes
- For larger organizations than the ASL, lobbying on behalf of the field and giving professional advice
The conferences organized by these societies are "meetings" in the genuine (non-academic) sense: they are an arranged time and location for members of the society to gather and confer, like a family reunion. The conferences are arranged by committees from the society, rather than by independent organizers, and the general expectation (and reality) is that the majority of attendees are members of the society.
These conferences are not like a car show where the goal is to draw in a large group of otherwise unknown people. The conferences are usually open to the public (with registration), but the general public is not the main audience - the members are.
This is where the bullet points in the question go astray, in my opinion: they assume that the main goal of the conference is to attract non-members to attend, when in reality the conferences were created to advance the purposes of the society and provide the society members an opportunity to confer and present their work. If an insufficient number of researchers thought that was worth the membership fee, the society and its conferences would disappear.