This has never been the case in any company I've ever worked at. Even if you can finish your day's work in, say, 4 hours, you can't just dip out for the other 4 hours of the day.
Managers and teammates expect you to be available at the drop of a hat for meetings, incidents, random questions, "emergencies", etc.
Most jobs I've worked at eventually devolve into something like "Well, I've finished what I wanted to finish today. I could either stare at my monitor for the rest of the day waiting for something to happen, or I could go find some other work to do. Guess I'll go find some other work to do since that's slightly less miserable".
You also have to delicately "hide" the fact that you can finish your work significantly faster than expected. Otherwise the expectations of you change and you just get assigned more work to do.