Why isn't there an option to leave the conversation?

That is weird, though this may be why I found this after doing some searching. If the "Leave this Conversation" option is not shown, it means someone in the group text does not have iMessage on or isn't running the latest version of iOS. If that's the case, you won't be able to leave the conversation. The workaround is either to delete the message or mute notifications by selecting "Hide Alerts."

When you first open iMessage click on your group and then at the top it should show the number of people and then click that arrow next to the number of people and click info then scroll down and click leave conversation.

Thanks