This is because in

const createUser = (name, email) => ({name, email})
const user1 = createUser("User Test", "test@test.com")
console.log(user1)

function is returning an object. This snippet is similar to

const createUser = (name, email) => { return {name, email}}
const user1 = createUser("User Test", "test@test.com")
console.log(user1)

Whereas in the second snippet, the function is not returning anything. It is a simple arrow function with no return value which is why console.log is undefined.

const createUser = (name, email) => {name, email}
const user1 = createUser("User Test", "test@test.com")
console.log(user1)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions#advanced_syntax

In an arrow function, to return an object literal expression requires parentheses around expression.

Therefore, your first arrow function has the correct syntax for returning an object literal, and the second one does not.