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.
Which is funny, since until relatively recently DRAM was what you produced in fabs with processes that weren't competitive enough for CPUs anymore.
There's a bunch of chinese DRAM companies currently playing catchup to get closer to modern densities. Could Apple buy one of those? I'm guessing there would be regulatory hurdles to that on both sides of the pond.