Why is competition limited in an oligopoly

An oligopoly is a kind of business wherein a surprisingly few numbers of producers produce nearly the whole supply of goods used by the people. Establishing up a brand and inciting enormous set-up expenses such as installing up production plants and mutable expenses like wages of workers related to different brands is very very hard as it requires billions of dollars.

the main idea of monster inc is good and evil

if im wrong correct me but have a blessed day!