If you're a prescriptivist, then open-source should be hyphenated for the same reason full-time, user-friendly, long-term, etc. are. That's how English works. You don't make the rules, and neither does OSI.
If you're a descriptivist, then why are is OSI getting a monopoly on everyone's vocabulary? You should be happy to let people use whatever term however they want. You know, freedom and all. You still don't make the rules, and neither does OSI.
MW also thinks it's hyphenated, but what do they know, right? https://www.merriam-webster.com/dictionary/open-source