Magento is headless because there was a constant demand for the separation between the front and back end for creating better marketing campaigns and next-level personalization. Now, the new technologies support decoupling between the front and back ends. But not all Magento stores are headless. Making a headless Magento is a choice. Magento stores are not headless by default. The second reason Magento is headless is that WordPress, Shopify, & BigCommerce already went big with headlessness. Magento had to keep up with the competition.