Asus doesn't make RAM. That's the whole problem: there are plenty of RAM retail brands, but they are all just selling products that originate from only a couple of actual fabs.
And a couple of smaller ones: CXMT (if you’re not afraid of the sanctions), Nanya, and a few others with older technology
If I recall correctly, RAM is even more niche and specialized than the (already quite specialized) general chip manufacturing. The structure is super-duper regular, just a big grid of cells, so it is super-duper optimized.
You’re correct that DRAM is a very specialized process. The bit cell capacitors are a trench type that is uncommon in the general industry, so the major logic fabs would have a fairly uphill battle to become competitive (they also have no desire to enter the DRAM market in general).