It’s Cheaper To Buy Two Sets Of DIMMs Than Larger, More Expensive Sets
Even though you are buying two of the exact same sets, there is no guarantee that they will work together. The DRAM that goes into a package has been tested to work together. Manufacturers don’t guarantee mixing or adding one set to another, even when they are the same exact model.
Customers often try this with higher data rate sticks using XMP to set them up. With XMP enabled, the motherboard may read the profile for two sticks of DRAM and set the secondary timings accordingly, but the tRFC timing for two modules may be 226, whereas four sticks require 314. This can be especially hard for most users to troubleshoot/find, as few ever go into the secondary DRAM timings.