OpenAI has seen a 'resurgence' of interest in secondary markets.

Bloomberg/Getty Images Since Anthropic and OpenAI have yet to IPO, most investors buy stock on secondary markets. For most of this year, investors wanted only shares of Anthropic, with almost no interest in OpenAI. While Anthropic still remains the clear favorite, OpenAI has seen a considerable uptick in demand. For most of this year, investors couldn't get enough of Anthropic while largely ignoring OpenAI. Now the tide is turning: although Anthropic remains the clear favorite, OpenAI is…