If you twist that ever so slightly, you have “I can’t afford it” being the responsible thing to say, since it shows that you know your limits. And you have “How can I afford it?” being the start of a self-delusion trip into debt, because you show yourself that you can afford the monthly payments for whatever.

“I can’t afford it” isn’t an automatic excuse to let you off the hook. If it is an excuse at all, it’s only one as far as “How can I afford it?” is an excuse for indulging in flimsy justification of unnecessary expense.