Part of the problem is that any given cooler doesn’t have a single W/K value. It depends on the temperature gradient across the heatsink which depends on airflow over the heatsink (and both the heat and heat density of the CPU being cooled), and the airflow depends on the case (and fans). And airflow temperature matters too (are you cooling the CPU’s heatsink with air that’s already been warmed by a GPU?)

Note as well that I said heat density - different benchmarks (or applications) will load the CPU differently, and the same amount of power consumption will result in very different heat outputs (heavy single core use will result in part of the die and heat spreader getting really hot, whereas heavy multi core use will result in the whole die and heatspreader getting kinda hot). If the heatsink has a very high surface area and high airflow it might have the ability to dissipate quite a lot of heat, but if the contact surface sucks (or is aluminum) it might not actually be able to transfer heat away very effectively without a massive temperature delta - resulting in high CPU temps for single core use even with a high cooling capacity for multi core use, for example.

Your average person can’t understand all of these variables, and the average reviewer can’t control for all of them. So, “this CPU with these benchmarks in this case with this fan configuration” is the best you can really expect. Because otherwise, what reporting are you actually going to do? “This cooler with 3.2CFM of 26C airflow on a CPU dissipating 80W on one core and 10W on the other 7 cores results in a peak core temperature of 87°C and a package temperature of 81°C” is not useful reporting. Especially because, when the average person doesn’t understand it, they’re gonna buy a case fan that says 3.2CFM on the box and then wonder why their CPU’s throttling.

Sure, but the goal of science is to distill these variables down into a model of what matters. Define a reference contact patch that's supplying the heat, and you've suspended all of the concerns about single core vs multi core. Then for more in depth articles, there could be a few to represent single core (smaller area), multi core (consumer processor), and multi core (larger server packages). Same with the airflow over the secondary parts of a cooler.

Another in depth article could then even analyze CPUs and how they themselves differ from that reference contact patch, and their own thermal resistance from each core to their surface.

And yeah, I know this is more work. The point is that doing the work allows the results to be used and compared across articles, even when some things are subtly different (eg contact patch). The problem is we're missing a strong incentive for these things to be objectively comparable, rather than having to read article after article and getting tripped up in the details.

The incentives aren't there (in my opinion) because the average person doesn't actually want to see that level of info. They're reading (or more likely, skimming) these sorts of reviews to have someone tell them what they should buy. The reviewer could go into a crazy amount of detail to figure out all of the thermal characteristics of the thing they're reviewing, but in the end it won't make a lick of difference compared to doing "just enough" and recommending something "good enough." Especially when, in most cases, coolers on the market these days are either "good enough or better" or "junk" and it's not hard to tell them apart.

There are exceptions to the above, like GamersNexus, who do take a slightly more scientific approach to things, but at the end of the day, most people don't care.

It also doesn't matter except for the most extreme overclockers. Even the worst tests are granular enough to tell you which ones are terrible and which are competent. The rapidly diminishing returns when you aim for the top 10% of performance in CPU building seems to hold here. Plus Steam's hardware survey puts the vast majority of users within factory overclock ranges. With so much of tech 'journalism' funded by ad views, you'll rarely get anything like extremely rigid methodologies outside of a few rare uber-nerds with dedicated followings (eg Igor's Lab, Snark's Domain) simply because it doesn't matter if, for eg, your 5700X3D is 70C or 73C under load.

Indeed, also, the variation between individual applications is enough to invalidate the results of the number of tests isn't large. It is super easy to mess this up. Too much paste, too little paste, a little bit of grease left behind or maybe a small piece of dirt... A nice way to practice application of paste is to use a piece of glass instead of an aluminum cooler. That will show you exactly what happens between the piece of aluminum and the CPU and it can be quite revealing how hard it is to get a perfect layer without any air trapped. Once you can do it perfectly with the piece of glass you'll be in a much better position to lay down a nice layer with the real thing.