I have two counter-examples for you: Georgia and Hungary. Both can be played as a newbie, but itsn't it sad that you might play them without playing their strength? I'm sorry but levying armies is not the simplest gameplay, and carefully waiting to have a dominant religion as Georgia before sending envoys, and converting city-States beforehand, requires preparation than a newbie, already overwhelmed with natural disasters, loyalty issues, gold deficit, raging barbarians and Waltzing Matilda, couldn't handle it.
It's not because here we're in an echo chamber of fans and thus people with enough knowledge of this game to analyze it, that everybody is like us. We see enough post of r/civ of people asking basic questions about district placement to know that a lot of people are still grasping to understand.
Do you think those people, who are stil pondering on a lot of thing, also know how to carefully manage their cities to be esctatic while looming for wars of liberation (Scotland), carefully placing your great works and playing with entertainment complexes (Eleanor), carefully being behind other civs while spawning in the tundra (Russia), getting to place your pyramids just in the right place to maximize yields (Nubia), choosing between building a mine over iron or rushing Jebel Barka (China), surviving the first era while all you have is unproductive, slowing jungle (Brazil), looking for where you can put the very restrictive polders (Netherlands)? We need to know how to walk before knowing how to run.
We need basic civs so that people can still learn the basic things of the game (district placement, spies, wonders, natural disasters, warfare, diplomacy), so they must be easy to use (so that it cannot be forgotten accidentally by an overwhelmed newbie), powerful enough so that a basic player can learn the rest without being bullied by the AI. There is no tutorial in Civ (the tutorial are just tooltips and the humongous Civilopedia), so we need to have "tutorial" civs, civs that are simple but potent: Rome, Greece, Korea...
I know it's the trend among "hard-core" player to feel superior about casuals and newbies, but remember we all started here, and a newbie that doesn't feel he's using a civ at its full potential because he doesn't has the level, or loosing because he tried to use the complicated abilities in a clumsy and self-destroying way, then it will not be fun for him, and it will restrict people trying this game. It's in the same time a capitalist way of thinking (more players = more consumers = more money), but also an inclusive way of creating a game.
Also, let's be honest: how many leaders do we have? 58. How many are "tutorial" civs? I'll say five: Trajan, Seondeok, Pericles, China Kublai and Simon Bolivar. Less than 10% of all the civilizations are "basic", civs that a newbie can handle without difficulties and without missing anything from the civ so he doesn't feal "cheated" for trying something he finally didn't used.