A single key is a single point of failure
Every externally-owned account, the kind you control in MetaMask or any standard wallet, is secured by exactly one private key. Lose that key and every asset it controls is gone permanently, there's no password reset, no customer support line, no recovery mechanism built into the protocol. Leak that key, and an attacker has instant, total, irreversible control. For a system meant to hold meaningful value, tying everything to a single unrecoverable secret is a strikingly fragile design, and it's not a minor inconvenience, it's the single largest cause of permanently lost funds in Ethereum's history.
EOAs are also rigid in ways that have nothing to do with security. An EOA can't pay gas in anything but ETH, so a user holding only an ERC-20 token can't transact at all until they first acquire ETH some other way. An EOA can't batch multiple actions into one atomic step, approving a token and then swapping it are two separate transactions, two separate signatures, and two chances for something to go wrong in between. And an EOA can't define custom rules of any kind, no spending limits, no multi-party approval, no automatic session permissions. Anything you want it to do beyond 'sign one transaction with one key' simply isn't possible at the protocol level.