Try spending just a single day where you only refer to yourself in first-person plural. Imagine telling your mom, "We're hungry for some dinner and we feel that ordering a pizza would be best for us." That would get old fast.

Kings and popes had to speak and write like that in public all. the. time. In the Magna Carta it was particularly important that things were phrased in the royal we because the barons wanted to be clear that the king was actually agreeing to it.

They wanted everybody to understand that King John himself was saying, "We promise to stop all the taxin' and start all the justice-in'," so that he'd have one less way to weasel out of it later…which of course he still did anyway.