They are rare
Celebrities represent a very small percentage of a normal population. Because they are very rare, they get paid a large sum of money.
The entertainment industries are based on them
Many entertainment industries are directly based on those celebrities. A movie, for example, requires good actors, and as a result, the actors get a good share of the money.
The entertainment industry is very profitable
The entertainment industry can be very profitable and can have very high margins. Some movies might gross hundreds of millions of dollars, and as a result, the actors get heavily compensated.
They influence a very large number of people
Because celebrities influence a very large number of people, they get paid higher. A celebrity can convince a very large number of people to use a certain product after appearing in a commercial for that product.
They get many opportunities
Once a person becomes famous, they can get so many opportunities to get paid in different ways. A celebrity can get paid to promote a certain brand, to appear on a certain show or to participate in a certain activity. (See Why do celebrities use Instagram?)
The are low in supply
Talented celebrities, such as talented singers or football players, are usually low in supply. According to the laws of supply and demand, celebrities should get paid more since they are low in supply.
There is a high demand for the industry
The entertainment industry is a highly demanded industry. The majority of people want to get some kind of entertainment during their day. Because the industry is highly demanded, it pays the ones who support it well. (See Why do Celebrities like Twitter?)
They win championships
Famous athletes can win championships and so bring glory to a whole country. Those athletes might get paid a large sum of money, especially if the sport they play is popular in their country.
People spend a lot on entertainment
People spend a lot of money on entertainment. This money goes back to the industry and a good portion of it reaches the celebrities involved.
They entertain large crowds
A single celebrity can entertain a large amount of people. A single singer can sing a song that gets heard by millions of people. This is one reason why celebrities might get paid a lot.
Why celebrities earn so much money?
Why do celebrities make so much money? Why do actors get paid so much?