Why do countries and cultures struggle to maintain spatial cohesiveness and national identity?

Cohesion is defined as a bond that unites members of a group together. working as a team to meet goals and care for the needs of one another. on the large scale of a country,there are different backgrounds, education levels, and social standings to deal with as well. This makes it extremely difficult to have a universal goal. Along with this you also have many different cultures coming together, trying to identify with one another, and reach a common ground of beliefs and ideologies.