> it seems they'd be a prime candidate to spin up their own memory fab.

While Apple et al certainly have the money to tilt up their own fab, they're savvy enough to understand the memory market's long history of constant boom/bust cycles. I still remember the huge DRAM shortage in late 80s forcing my startup at the time to delay launching our new product for a year.

People assume Apple cares about vertically integrating cost but they're actually focused on integrating margin. Apple has billions in cash on hand and when they think about what to do with it, a key metric is Return on Capital, especially the margin that capital will generate. Since a core metric public companies are judged on is blended margin, they are looking for ways their bags o' cash can be put to work generating revenue at margins that will pull their current average margin up vs down.

Averaged over time, mainstream memory devices are historically one of the worst margin areas of the semi market. It's super expensive to tilt up a fab on a new node but once you do, turning the crank faster to make a lot more chips isn't too hard because mainstream DRAM tends to be quite uniform. So when a fab on a new node and/or RAM generation first opens, the margins tend to be pretty great. But as the node matures and/or the RAM generation goes from 'new' to 'commodity', competition heats up as everyone gets better at making more faster. Then they're tempted to maximize revenue by cutting prices until their mature fab is at 101% utilization. And that eventually drives margins down until someone's selling near cost to sustain their low-price-enabling volume - with occasional dips below cost when they get stuck holding excess inventory. That's why cash-rich companies with high margins like Apple are delighted to buy DRAM built with Other People's Money. As long as the DRAM market is under competitive pressure, Apple gets to shop their huge orders around to get the absolute lowest price on RAM that was built with other investor's low margin dollars.