As a consultant, you get to decide how often you take jobs. You can work yourself to the bone, or you can take 6 months off per year. You can change your work schedule as your mood or personal situation changes. If you're up for it, it's quite easy to work your ass off, and it's quite easy to turn that efficiency into more money. And when you're young and/or strongly financially motivated that's easier to do.

As a salaried employee, you usually can't control when you work. You're (supposedly) going to be in the office ~40 hours a week for ~50 weeks a year, every year. So you naturally shift into a slower pace to avoid burnout.

I don't see it as a problem. You trade off efficiency for long-term stability.