Primarily because the release is the most expensive and most expendable technological gadget that most compound shooters decide to saddle themselves with. Sights, rests and even stabilizers are all far more beneficial than that release. Of those, the arrow rest is the only critical, must have part that you cannot do without on a compound.

So, for someone that wants to simplify their setup, the release is logically the first thing to go. With a little practice, your fingers are quite capable of doing the job. Yes, a release gives you a bit better accuracy in absolute terms. But I've seen very few hunting shots where 1/4" in accuracy at 20 yards made the difference between a quick kill and a wound. To put it in 3D terms, it doesn't matter whether the arrow is sticking dead center or off by the line, a hit in the 11 ring still scores an 11.

I shot a release, of and on, for about 5 years. Now, I shoot far more than the average bowhunter does but, over those 5 years, I had one release break and two others malfunction. Going to my backup releases, I found NONE of them shot exactly the same as my primary release. Even identical releases shot to a slightly different point of impact. So, someone that carries a backup release needs to practice with it enough to learn how it shoots before relying on it in the woods.

Frankly, as long as I've got functional fingers on my stringhand, I'd rather micturate in my own mug of beer than ever shoot a release again.