This is under the assumption that there is a surplus in society that can satisfy the needs of everyone. Marx’s point is that technological development and industrialization could make this possible. As such, the need to motivate people to work harder is not necessary.
Prior to such a surplus existing, the distribution of goods would be more akin to “From each according to their ability, to each according their contribution”. That ensures people are motivated to maximize their productivity as long as that’s still necessary.