They achieved AGI internally, but didn't want OpenAI to have it. All the important people will move to another company, following Sam, and OpenAI is left with nothing more than a rotting GPT.
They planned all this from the start, which is why Sam didn't care about equity or long-term finances. They spent all the money in this one-shot gamble to achieve AGI, which can be reimplemented at another company. Legally it's not IP theft, because it's just code which can be memorized and rewritten.
Sam got himself fired intentionally, which gives him and his followers a plausible cover story for moving to another company and continuing the work there. I'm expecting that all researchers from OpenAI will follow Sam.