He have created the asset that revolutionized the financial market forever does not mean that is good in all the areas. A exchange is completely different. Demands management of people, security, technology, informations and taxes. Besides that, he would have to implement KYC, something that goes against the decentralized nature of which he is adept. Maybe a wallet would stay more in his area, by the knowledge that has in technology. Would be a good wallet? Yes, I would buy a wallet made by the own Satoshi.
Don’t also forget that we have something called DEX decentralized exchanges, so even if he created one it would have taken the nature of decentralization.
Moreover, I love the fact he stop where he did because he didn’t have to do all the jobs himself, created it or started it and let the world do the finishing and make whatever that will be required for much easier access.