While I'm not privy to OpenAI's internal motivations, it's worth keeping in mind that as a nonprofit they face a pair of hard organizational constraints:
1) Training state-of-the-art ML models costs huge amounts of money. That means OpenAI's capex is forcibly high.
2) Their charter (https://openai.com/charter/) commits them to a "fiduciary duty to humanity", so among other things they can't accept donations which are subject to conditions that may be misaligned with their mission. That means OpenAI's effective funding pool is forcibly low.
The combination of 1) and 2) means that, absent an absolutely massive yet hands-off private donor — a rare thing indeed in the nonprofit world — it's hard to imagine a solution to this dilemma that doesn't involve some kind of for-profit spinoff. And the outcome here really is pretty binary: if you believe OpenAI will build safe AGI, then 100x returns for investors really are immaterial compared to the wealth they can expect to generate for everyone else. While if you don't believe they'll build safe AGI, then you must either believe that A) they'll fail to build AGI (in which case investors get nothing anyway); or that B) they'll fail to build it safely (in which case your concerns are presumably technical rather than organizational).
Incidentally, it's quite likely that in the beginning, OpenAI's own researchers were by no means sure they'd be able to get results like GPT-3 through scaling alone. But that turned out to be the world we live in, so they're now playing the hand they've been dealt.