Money, and there's enough demand for them to turn a profit. They especially charge more cash for the very niche 420mm. [I was interested in them at first, because 140mm fans, but the limited case options was a major con.]
The larger sizes don't offer much beyond fans, and more fans or bigger fans isn't always more better.
Some review benchmarks be way off the mark. One of the biggest mistakes I've seen with them, IMO, is the cpu choice(s), or not utilizing a dummy heater. I'd think most want to see what the cooler can really do, so the reviewer should use a dummy heater or a 'big' cpu and stress test that. [Maybe they don't have access to such samples - I could cut 'em some slack for that.]
Intel's X-series is one of the best samples to use if a dummy heater wasn't available.
Others... I've seen quad core i5s - i7s(the Intel TIM wasn't helping), and later AMD Ryzen cpus, which don't really pull that much power, but are harder to cool due to the multi-chip design and higher thermal density of said chips.
You can't really see what these really big coolers are/aren't capable of with chips like those. Users can't saturate those radiators with chips like those - they're going to run into other limits long before that.
Trying not to make the post too long, but more cooler reviews need to be done with i9s and dummy heaters... and I guess Ryzen shouldn't be left completely out of the loop.
Then viewers need to make their own judgements whether they 'need/want' such a cooler.
I do kinda regret getting 2 360mm AIOs, but my curiosity had gotten the better of me at the time. [I do have one 280mm on the side as a backup.]