Because it isn't how people normally use cards so the reality is that most people won't do it. Having to interact with a card beyond simply handing it over to pay is an unacceptable level of faff and most people despite the best intentions simply won't do it. I mean seriously, you see no issue with having to remember to enable and disable your card all the time?
With a proper credit card you simply use it normally and are covered by the legal fraud protection measures in the unlikely event of anything going wrong.
No need to mess about topping up either.
Who said all the time. You do it when you want or not at all. And no i see no issue with it at all.
I use the supercard and use it just like a normal card you get full protection as well. I see no reason why you would faff around with a Clarity which needs paying off a few times a week to avoid paying interest.