Monolithic applications are hard to scale. Some parts of an application, such as databases, are next to impossible to scale horizontally - which means the entire monolith may not be scalable. Scaling vertically is not always an option due to the physical limitations of the servers and the costs.
This issue, along with several others, can be resolved by splitting monolithic applications into microservices. Microservices can be scaled up or out independently based on the requirements of each service.