Why do sociologist say that race is socially constructed

Because they have defining features such as skin color and physical capabilities.

Social Stratification is when we sort other skin colored people into a hierarchy just to justify inequality. I personally think that is rude and unfair.

The concept of race is ubiquitous yet tricky : people think race is obvious because, if you are visually abled you can tell that people are different based on their skin colour and other physical signs