That's because the airwaves are public, so the FCC could regulate / require equal time in the public interest. A TV or radio station isn't operating entirely on its own dime - it's using a public resource, so it should be done in the public interest.

There was no fairness doctrine applied to, say, newspapers.

The internet is a public resource.

It is more a massive collection of the different resources of many different people.

Not in the same way the airwaves are. You can run your own network cable but everyone has to share the same limited airwaves.

Right, I meant they're not bound by the first amendment. There are plenty of other regulations that apply to specific outlets, but they're not rules to protect free speech