I think Windows PC makers sell PCs with 1.5GB, 3GB, 5GB, and 6GB of RAM to try to use all their 512MB, 1GB, and 2GB RAM module sticks which maybe taking up a lot of warehouse space at their PC factory, and PC parts storage warehouses.
After companies use all their 512MB, 1GB, and 2GB RAM modules, they have more warehouse space for more popular RAM module stick sizes like 4GB, 8GB, and 16GB RAM module stick sizes.
PC companies can also sell their computers for an extra $20 or more when they add slightly more RAM to their PC. Adding slighty more RAM to a slow PC makes it looks like a better deal by bundling an extra 512MB, 1GB, 2GB of RAM on a cheaper computer, and may make cheap computers sell faster.
Too much competition, combined with an audience that doesn't see any difference other than the price and the way a given PC looks like.
Take Apple for example, they can easily make profit off much more expensive hardware, because their audience lies mainly in the creativity sector.
And now there are new PC makers on the raise that make Linux laptops such as System76, Purism, and Station X.
Each one of them successfully sell laptops at a higher price than Windows PC makers sell, mainly because they have built a good reputition within the Linux community, Linux users are usually more experienced with tech (not necessarily a fact, I just mean the average user), we generally know the differences in specs goes beyond just a price tag.
Then there's the fact that Microsoft keeps trying to block Linux away from the mainstream brand laptops, and more and more new Linux users aren't tech savvy enough to program their own drivers, so we're now more willing to spend more money on a laptop we can immediately use than ever.
I think PC with uncommon sizes of RAM will make fewer people want to buy a PC if it comes with a less common amount of RAM like 3GB.
Many users will rather buy a slightly cheaper PC with 2GB of RAM, or slightly more expensive PC with 4GB of RAM. Plus, PCs with 8GB of RAM are becoming more affordable where a PC with 8GB of RAM cost $400 or less.
