What incentives do people have to create echo chambers?

The perception that disagreement is bad.

Seems to me that if two people agree perfectly on something; there's nothing to discuss and no reason to communicate.

Where people disagree, then a discussion can happen, and others can be entertained and informed even if the original parties aren't.

If I feel the need to say something, and can't stand the thought of contradiction, I'll go shout at the trees some more. They may not agree, but they don't talk back.

Social validation. It is good to feel that you are right and it is good when other people tell you so or post stuff that validates your believes.

In your chambers you learn the arguments to defend your ideas and fight the others. Tribalism.

What else is there for businesses? Profit! It is the one true incentive.

Comfort, and validating your worldview.