Question

Why do people form interest groups?

Answer 1

Interest groups attempt to influence public policy in a variety of ways. ... Interest groups also organize lobbying efforts to persuade members of Congress to support or oppose particular pieces of legislation. They also organize group members and mobilize them to influence the Congress.

Answer 2

Interest groups are usually formally organized, on the basis of one or more shared concern about something, attempts to influence people about it.
i hope this helps.