Because they're either derp or too lazy to swap out their Evisceration set.
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Wheres the source for this testing?
Here. There isn't a lot of data points yet but even very limited testing shows that there's a significant drop with each TH level increase.
Also keep in mind that if you're trying to increase TH level, sneak attacks and trick attacks are your best bet (Not thoroughly tested yet, but pretty much all of us can attest to its effect), and removing your TH pieces when you're doing it is going to reduce your TH procs pretty substantially, which was the entire point of the post you were having doubts about. If you're mostly there just for TH you're "wasting" your SA and TAs if you decide to not wear your TH gears while you're doing it.