The main reasons why you can't leave a group chat are:
- If the group has only 3 members, iMessage does not allow anyone to leave because leaving would reduce the chat to a one-on-one conversation rather than a group. Apple requires 4 or more members in the group to enable the "Leave this Conversation" option.
- If the group chat includes people using non-Apple devices (such as Android phones) and is SMS/MMS-based rather than an iMessage group, you cannot leave the conversation. In such cases, the group chat behaves like an SMS conversation, and the option to leave is not provided. You would only be able to mute the conversation or block it.
- Rarely, technical glitches or bugs can cause the "Leave this Conversation" option to not appear, which can be resolved by restarting the Messages app or rebooting the device.
To summarize, you can't leave a group chat if it has fewer than four members or if any participant uses non-iMessage messaging. The workaround for three- person groups is to add another member to make four, then leave. For SMS groups, muting or blocking the conversation is the only option.
