Because a caffeine headache is neither a migraine nor a tension headache. And according to Wikipedia, paracetamol is mainly only useful for migraines. For tens

Because a caffeine headache is neither a migraine nor a tension headache. And according to Wikipedia, paracetamol is mainly only useful for migraines. For tension headaches, you need the combination with caffeine and aspirin as well. Caffeine constricts blood vessels, which is why it's useful against headaches. And why withdrawal causes headaches, because now you have more blood flowing than you've been used to.