From my outsider perspective, it's pretty straightforward why they don't.
In Intel's case, there's ample coverage of the company's lack of direction and complacency on existing hardware, even as their competitors ate away at their moat, year after year. AMD with their EPYC chips taking datacenter share, Apple moving to in-house silicon for their entire product line, Qualcomm and Microsoft partnering with ongoing exploration of ARM solutions. A lack of competency in leadership over that time period has annihilated their lead in an industry they used to single-handedly dictate, and it's unlikely they'll recover that anytime soon. So in a sense, Intel cannot make a similar product, in a timely manner, that competes in this segment.
As for AMD, it's a bit more complicated. They're seeing pleasant success in their CPU lineup, and have all but thrown in the towel on higher-end GPUs. The industry has broadly rallied around CUDA instead of OpenCL or other alternatives, especially in the datacenter, and AMD realizes it's a fool's errand to try and compete directly there when it's a monopoly in practice. Instead of squandering capital to compete, they can just continue succeeding and working on their own moat in the areas they specialize in - mid-range GPUs for work and gaming, CPUs targeting consumers and datacenters, and APUs finding their way into game consoles, handhelds, and other consumer devices or Edge compute systems.
And that's just getting into the specifics of those two companies. The reality is that any vendor who hasn't already unveiled their own chips or accelerators is coming in at what's perceived to be the "top" of the bubble or market. They'd lack the capital or moat to really build themselves up as a proper competitor, and are more likely to just be acquired in the current regulatory environment (or lack thereof) for a quick payout to shareholders. There's a reason why the persistent rumor of Qualcomm purchasing part or whole of Intel just won't die: the x86 market is rather stagnant, churning out mediocre improvements YoY at growing pricepoints, while ARM and RISC chips continue to innovate on modern manufacturing processes and chip designs. The growth is not in x86, but a juggernaut like Qualcomm would be an ideal buyer for a "dying" or "completed" business like Intel's, where the only thing left to do is constantly iterate for diminishing returns.