Hello,
Simply because it cost a lot of money, time, resources, etc to be able to design, then build and sell a powerful gpu, taking also into account the present and future technological landscape.
Not all the gpu can be designed, often you have limited design capability, there are patent to take into account, then how many people, money you are throwing into the design and R&D phase.
The same apply to the manufacturing, you have limits on how big a gpu die could be, you have efficiency limits, how many good gpu dies come out from a whole piece of silicon. The manufacturing capabilities need also to be took into account, the technology to produce these gpu dies cost a ridiculous amount of money.
Finally you need to take into account the software and developer landscape, the games and programs that will use your gpu. Again this is not an easy task to match the requirement of the user and at the same time being really more powerful than everything else.
Hope it help you a bit to understand how hard it is to build a gpu, even harder a successful and powerful one.
I like tho your name proposition, RX-Eclipse sounds pretty awesome for a gpu name!! 
What it really comes down to, is that NVidia is a large operation that has the ability to produce multiple cores from a single architecture for different purposes. Back in the Fermi/Kepler days, NVidia added quite a few FP64 cores to their GPUs which gave a great deal of compute performance. Unfortunately FP64 cores a larger and more power intensive than FP32, so when playing games (which use very little FP64), there you can fit less cores on a die and the run much hotter than FP32 in gaming workloads. So the Fermi/Kepler GPU dies were massive and inefficient when compared to the Radeon 5800 series, and worse yet, workstation users started buying gaming cards instead of the much higher margin professional line.
So with Maxwell, and ultimately Pascal as well, NVidia stripped out a lot of the FP64 functionality in gaming cards, saving that for the Tesla line. That allowed them to have drastically better power efficiency, more cores, and higher clocks.
AMD on the other hand, doesn't have the resources to develop individual workstation and gaming cores. So they are stuck developing one core that can be used for both. RX Vega has twice the FP64 performance over the GTX 1080Ti, and it shows in the compute performance.
This allows AMD to use the GPU for machine learning/professional rendering/AI applications as well as gaming. But naturally, those extra FP64 cores make Vega less efficient at strongly FP32 tasks like gaming. To help combat that, AMD created the Mantle API, which strongly leverages compute functionality to deliver better gaming performance. A lot of that functionality was then built into the DX12 and especially the Vulkan spec which is why those specs tend to perform better on AMD hardware.
NVidia, reticent to add strong FP64 back into consumer cards to keep their professional margins intact, created Gameworks. A means of doing a lot of the compute style graphics enhancements within a standard FP32 workload. Naturally then those enhancements perform better on NVidia hardware.
Game developers have been slow to add a lot of the compute functionality to games, simply because of NVidia's much higher install base and market clout, in push Gameworks onto developers. AMD has had some luck working with developer Bethesda in using Vulkan in their games like Doom, and Wolfenstein II
You can see that when developers make use of those extra FP64 cores on the Vega 64, it's gaming performance rises relative to the competition. But bottom line, gaming will likely remain a strongly FP32 workload for some time, and NVidia will have a advantage here by their ability to fab out FP32 cores for gaming, while saving the designs with more FP64 cores for their Tesla line. The advantage Vega has, is that it can do both reasonabily well. It can game in the GTX 1080 ballpark, while climbing to 1808 Ti levels with Vulkan support. And while not as strong at compute as NVidia's workstation cards, it is worlds cheaper.
So in making the GPU you request, would that be all FP32 cores and just a gaming behemoth? Then what will AMD do in the professional space? Just cede that entire market to NVidia? Or if you include FP64 cores in your design, the GPU would be so large and hot, it would be difficult to keep the clocks up for decent gaming performance. Sure it would be nice if they could two separate cores the way NVidia does, but they simply don't have the resources for that. Such is life when you're the little guy.