Because the assumption is it's a valid charge but the user simply mistyped their CVC (or don't understand what that number is and put in the wrong thing entirely). If these purchases are rejected, the user may never complete the sale.
I was a bit shocked to see this behavior at first and spent some time debugging it thinking it could be an issue on my side. It turned out not. It's no big deal but could bring doubts on consumer's mind if they intentionally type wrong CVC (I used to do that before on other websites!). Glad to hear that Stripe is on it and can't wait for that tool to show up.
Can you please also answer a few of my other questions including the reliability seal thing? I'm sure that would be a simple change from your side and the developers may not even have to do a thing.
Some users want that behavior. But we're rolling out tools that give you the option to decline charges if the CVC fails.