I've quit a couple companies, but never because I was bored. Usually it's because I'm frustrated.

In my experience, people don't leave because they're bored, and the company doesn't know. People leave because they're frustrated, and they repeatedly tell their managers (who don't do anything about it).

When a person says "We can improve the company by doing X!" two or three times, and management doesn't let them do it, they're going to realize that their continued employment there is pointless, and quit. This is worse than boredom. When one person is bored and quits, that doesn't necessarily infect their coworkers, because different people are interested by different kinds of things. But when one person can't get anything accomplished because of bad management, everybody sees that as a problem.

I've seen a person quit, and then the entire rest of the team follow in the next few days. They had a fascinating project that everybody loved, but were being frustrated with bad tools, bad office environment, changing requirements, and scope creep. Once the team lead was gone, there was little reason to stay.

A single bored person is bad. A frustrated team is infinitely worse.