Social platforms decide how much of your audience sees a post. Reach changes, the favoured format changes, ad costs change, and none of the rules are yours. You can build an audience of ten thousand people and then be shown to four hundred of them, for reasons nobody will explain to you.
Email is different. Deliverability still matters, and mailbox providers still make decisions about placement, but the relationship is direct. Your list is an audience you hold, can segment, can measure and can email tomorrow without asking permission or paying to reach.
That distinction is the whole argument. One is rented, one is owned.