Sadly, your assumption is incorrect. Some of the time, as a merchant, you do get a specific indication of problems like insufficient funds or an expired card. The problem we're talking about here is all the other failures, where you just get some entirely unhelpful generic code that translates roughly to "declined, for reasons we're not going to tell you". From personal experience, this second group can often represent the much larger share of total failures.
When a generic code comes back, you don't get your money, but in a not insignificant number of cases, you do get an upset customer contacting you to ask why their payment didn't go through when their card should be fine and they've been using it with you for a year now and they just used it this morning at their local store with no problems. You can understand their frustration.
The reality is that most people don't realise how horribly unreliable the card payment infrastructure we rely on actually is. They can and will blame the merchant who declined their card if they don't know of any reason that it shouldn't have worked. Even if they don't blame the merchant explicitly, having their card declined causes a lot of people some embarrassment, and that is not a reaction you want associated with your store or service if neither you nor your customer has done anything wrong.
Disclaimer: I'm not familiar with these specific numbers.
Anecdotally I've had my own cards denied because I've reported my card missing and had to get new numbers. This is fairly common from what I've seen, many people I know have had issues because of this (late fees, etc.). Just another case I thought I'd add, as we sufficient funds and it wasn't fraud.
