We say “I will do that later” because we do not want to say “yes” or “no” now.
Oh, we have plenty of logic to explain why it makes sense to postpone action. But beware. Explanations are usually not explanations based in reality. Explanations are usually excuses arising from internal confusion. From a lack of courage.
Do a quick word substitution in your head. Anytime you say “I can’t” say instead “I choose not to” and then sit and see what bubbles up.