I get such a kick watching consumers guessing about what happens behind the scenes at the OEMs in new product development. I actually have worked as a product manage at several large OEMs on the server side and can tell you that there are many factors at play when developing a new product. Its far from the cut and dry decision making you think it should be.
These factors include (but are not limited too) historical sales data, market trend data (IDC), customer feedback, vendor roadmaps (Intel/AMD/etc.), budgets, manpower, vendor NRE payments (meetcomp), and probably the biggest influencer - Politics.
OEMs are typically always short on manpower resources and budgets, so this has a lot to do with how many projects (new products) get developed because it takes a lot of resources and time to design a new platform. Now that isn't to say that the ODM/JDM doesn't help, they do. But they are in most cases only part of the equation. Budget/money is another huge factor. There typically is never enough so many OEM count on the vendor (like Intel) to absorb some of the development cost (meetcomp). This practice was started for the most part by Intel years ago to get more design wins from it's competitors. AMD somewhat follows this practice today, but given Intels huge financial resources they typically have much more to spend. So if your an OEM looking to get funding for a project, who do you think management is first going to turn too?
Yup, the good old Bank of Big Blue (Intel).
Also don't rule out lack of vision. Many OEMs suffer from what.Professor Clayton Christensen of MIT in the "The innovators dilemma" wrote about years ago about the problems that large OEM have with being able to see disruptive new markets opportunities due to their historical customer successes. They simply couldn't see the what was coming before them or it represented to little return in value and risk compared to maintaining the status quo, they avoided it. I've seen this myself time and time again.
Management also gets addicted to Intel's money and it takes a low risk path because its worked before in the past, the "no one ever gets fired for buying IBM" syndrome.
Problem is that when a disruptor (AMD) enters an established market, large OEMs are typically last to jump due to the perceived risk involved. This gives smaller players a big advantage to grow their market share. You see this being played out with AMD's Ryzen products. ASUS, MSI, Gigabyte and other small players jumped quickly to bring cutting edge products to market fast, where as large OEMs HP, Dell, Lenovo are just now beginning to role out a few (mostly lower end - low risk) products while flooding the marketing with 10th Gen Intel products. Why? Easy, its playing it safe. They know the Intel products will sell and they have the historical data to prove it. They got meetcomp $$$ to make them. Product refresh cycles are easier and often quicker to bring to market. So lots of new Intel products.
So why didn't they build equivalent AMD Ryzen products?
There are a number of reasons which can include the additional engineering effort to adding a new CPU architecture (Firmware, service, sales training, etc), lack of vendor funds (NRE meetcomp) to aid development, lack of historical customer data (We don't know how this would sell), internal political resistance (yes, there can be bias), desire to remain single vendor, product cannibalization, playing a wait-and-see (will AMD burn out or are they hear to stay?), etc. So it's difficult as a product manager to simply say "build it and they will come", in fact most product managers are no more than product influencers, they lack any of the decision making power; that comes from upper management and finance.
So, its not so simple.
There are many things at play and what may seem obvious to the consumer, isn't always obvious to the OEM. But you have the ultimate buying power. That speaks loudly. Using it to sway the OEMs by purchasing the kinds of products and service you want to see will eventually sink in (or they go out of business). If that means giving the smaller guys like ASUS/MSI/etc. your money before the big guys, so be it. Eventually the HP/Dells/Lenovos will figure it out.