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.