Why group messages expose everyone
A group text is a shared thread by design. The phone has to show every participant so replies can reach them, which means every number is visible to everyone in it.
iMessage, Android RCS, WhatsApp and GroupMe all work this way. Some hide numbers behind a display name for contacts people already have, but anyone can still see who is in the group and get in touch with them.
There is no privacy setting for this because it is not a setting. It is how a group thread functions.
What a broadcast does differently
The same message is sent as separate individual texts, one to each person. The recipient sees a message that arrived just for them, with no recipient list attached.
Because there is no shared thread, nobody can reply to everyone. A reply comes back to the sender only, which is usually the practical benefit people notice first.
It also means recipients cannot see how many people received it, which matters more often than you would expect. A guest does not learn the guest list, a client does not learn how many clients you have.
Where this actually matters
Any list that mixes circles. A wedding brings together two families and several friend groups who have never met, and there is no reason any of them should end up with each other's numbers.
Any list involving a business relationship. Clients of a salon or a garage have no reason to know who else uses it, and tenants in a building have no reason to be handed each other's contact details by their landlord.
And any situation where a reply-all would be awkward. A funeral announcement where thirty people reply with condolences to everyone is a real and avoidable problem. Reaching people about a service covers that case specifically.
The email comparison people expect
Most people arrive at this looking for a BCC field, because that is the mental model email gave them. Email has one, text does not.
The closest equivalent is what a broadcast does, and it is arguably cleaner than BCC, since a BCC recipient can still see who was in the To line and can still reply to whoever is visible.
The trade is that a broadcast is genuinely one-way. If you want a conversation, you want a group chat and you accept what comes with it.
What you give up
Conversation. Nobody can talk to each other, which is the entire point but also a real limitation if the group needed to coordinate between themselves.
You also cannot see a thread history with each person, since replies come back individually rather than into a managed inbox.
For most announcement-style messaging that is fine, and for anything social it is not. Plenty of people run both: a group chat for talking and a broadcast for the messages that have to be read. What changes when the list gets large covers the larger end.