Apple bought PA Semi as the starting point to getting off of Intel. Theoretically, memory seems like something Apple could figure out how to fab. And it's not like they don't have any capital reserves.
In the Tim Cook era when Apple needs to lock down the supply of a commodity part, they have a history of buying a dedicated manufacturing line for a manufacturing partner.
It's crazy to think Apple would actually fab memory (or TSMC for that matter). It's an entirely different process than logic.
They bought P.A. Semi, but it was for their design capability; they never had fabs anyway, and Apple still depends on TSMC and others for manufacturing chips. Apple building fabs to ensure a guaranteed supply of memory (or logic) chips would be an unprecedented level of vertical integration, even for them.