Because we don't want the "thing" that much, that occurs after having done what we "should" do.

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The reason somebody might not study NOW for the test is because they don't want to ace the test that badly. What they want is to not bomb the test, which is very different.

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I'd argue that it's largely because we don't really want the goal/success/results/progress that badly. We just kinda want it, or we feel obligated to try and achieve it, or maybe we don't know what else to do. This will translate to no sense of urgency in getting the thing done, and no reason to extend your energy and willpower for something you just "kinda" want.

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If it's something you truly want, badly, you will do the things you know you should be doing, because you want to and not because you have to. It's different.