"The internet" is just a bunch of computers connected across the world. The content of the internet is provided by the computers which are owned by millions of different people and organizations, but the infrastructure which connects them together is owned by businesses that operate and maintain them.

Someone who starts their own ISP isn't "creating internet" like growing corn or something, they are just running the communications lines necessary to connect their customers to a larger communications backbone relatively nearby. The companies which own and operate the big, high volume communications lines don't want to bother trying to deal with individuals so only collections of users under the umbrella of an ISP are worth providing access.