Not just Sustkever, but other top researchers joined the then nascent OpenAI team for the same reason. Most of them on-record indicating they turned down much bigger paychecks.
The problem I see is, astronomical costs of training and inference warrants a for-profit structure like the one Sam put up. It was a nice compromise, I thought; but of course, Sustkever thinks otherwise.
Maybe Sutskever is finished with his LLM experiments and now has other interests and ideas to pursue meanwhile Sam was keen to make money and stay on the same trajectory. Microsoft also felt the same way.
Seems reasonable, I mean that's why Sutskever joined in the first place ?