Oof... Where to even start...
Let's go with the simple one: not all gamers use their PCs the same way.
Outside of your regular Monster Hunters, Final Fantasy RPGs and MMORPGs, Call of Derps and so on, there's plenty games which will take mods, encourage the use of mods and even play in different ways depending on the input method you want. For example: driving simulators with all the bells and whistles you can get, which could include VR HMDs, "proper" Wheel+Pedal+Gearbox combinations or flight sim rigs. All those require way more CPU horsepower than your average "I watch YT while playing" gamer. I talk 100% from experience here and I can tell you, some CPUs that appear in charts as great "value" or even "great performers" fall flat when you need to put them in such scenarios. For that, you need to understand exactly where the CPU will land in situations where the graphical load is secondary to the CPU. Remember, the CPU is still in charge of inputs management and coordination at the end of the day. The GPU will have to wait for the CPU in a lot of situations where a combination of mods, different input types and, sometimes, I/O wait (there's some really interesting conversations to be had for networking and CPU dependency) will affect your overall framerate and experience.
I could say, for argument's sake: "hey, Tom's sucks at benchmarking because they're not using fully a built flight simulator with feedback cockpit and commands while using Starlink connection!". Your "I want 4K for my CPU tests" argument falls within strike distance of that one if we go by the Steam Survey and other somewhat respectable data that tracks monitor usage. Would it be nice if they did that? Absolutely. Is it realistic, or even valid to do so? I'd argue no.
Another easy one, like it's been discussed already, is longevity and GPU swaps: if you're planning to keep the motherboard, RAM (asterisks here) and CPU for a good while, then you will aim for the best CPU (let's call it platform) you can get and then the GPU as part of your budget. For throw-away builds (as I call them), then there is an argument to be made for having 4K testing handy.
Still, even then, you can always extrapolate from the combination of GPU and CPU testing. It's not an outlandish conclusion to say: "mid GPU + mid CPU = mid overall performance", yes? And you can go from there. Sometimes the data you have is enough to still make a valid and informed decision.
So, do you really understand why they test at 1080p?
Regards.