They have to keep up all the computing infrastructure running the simulation, and deal with long-term threats like geological activity and asteroids. That requires having and maintaining considerable technology out in the real world, and if only a mere 10% refuse to enter the simulation, then that minority may not get to benefit from spacefaring technology, or have the capital or industrial base to develop their own.

In fact, since spacefaring presents a threat (kinetic bombardment) to any planetary-bound species, then that majority who choose to enter the simulation may want to expressly prevent the minority from leaving the planet.