I suppose that there are many reasons for having an automatic transmission in a Corvette. Don't forget that the first Corvettes were ONLY available with an automatic (the Powerslide).
This also isn't a new trend. Dig out your copy of the Corvette Black book, and you'll see that transmission choice from manual to automatic started during the C3 era, and never went back.
Trivia question: other than the C1's, in what model year did the automatic transmission build quantity exceed the manual transmission build quantity? It's probably earlier than you think! -- answer below...
Although I drive a six speed '95 (and enjoy it), there are times when I would be just as happy with an auto. My better half has already indicated that when we buy a brand new, Museum Delivery Vette, that she would prefer the auto (and with the six speed auto, I think I won't have a problem with that).
I also suspect that the demographics of the car tend toward older buyers. I will also speculate that older buyers, on average, prefer more convenience and comfort options (like automatics) than younger buyers.
When I was younger (like when I had my '65), there was no way I would have wanted anything other than the manual. Now that I'm older, the six speed is fun (especially the sounds & the feel), but I would still enjoy the car with an automatic.
My wife and I are more into the social aspects of the hobby. I love talking about and driving the Vette, but I also enjoy the friendships I've made here and in my local Vette club. When I was younger, it was strictly about me and the car.
So, while I can appreciate your feeling that Corvettes should be manual, I can certainly see why most of them are not.
I also think it's poor form on your part to castigate other Corvette owners because of their choice in transmission. That seems a bit shallow to me - do you also believe that all Corvettes should have manual windows, manual steering & brakes, and convertible tops, too?
Trivia answer: 1972 14,543 automatics of 27,004 built