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.

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.

DRAM fabs are their own well-known specialized process which is covered by the DRAM companies. It doesn't make sense to start a competitor for it.

It's crazy to think Apple would actually fab memory (or TSMC for that matter). It's an entirely different process than logic.

They probably could, but time is a big factor.