From a small employer perspective, good employees do more than assigned. It's impossible to name and identify everything you want done within a certain amount of time. There is always more work than hours in the day to do it. Maybe corporate America is different (if so, I guess they're paying for redundancy, which isn't necessarily a bad thing).
I think that there is usually something more that can be found to do but most people generally have a set amount of productivity they are going to give you on any given day. Even your best employees are not giving you 8 hours. Anyone honest with themselves knows that they aren't dialed in 100% of the time that they are at work. There may be certain days or a stretch of days where that happens like as you approach the last few days before a deadline, but that is usually followed by a letdown the days following completion. They are still working and still giving their "8 hours" but it isn't like the 8 they were giving right before the deadline.
From my experience, though, those who say "but you said I only had to do this and I finished now I want to go home" are the ones you get rid of. You want people who understand their "job" is to make the firm successful and I'm paying you to do that 40 hrs a week. If you do a better job at it than others, then I'll pay you more. The best employees I've ever had understood this and did stuff all the time I had not assigned them. Other people I know who own small businesses have expressed the same sentiment.
Yes. The carrot I mentioned. Your best employees will take what you assigned and instead of completing it in Z time will do it in Y time and then help you with tasks A,B,C as long as there is an incentive to go above and beyond. However, even your best employees will start to weigh the compensation against what they are being asked to do. The smaller your business, the less coworkers your superstar has to compare themselves to.
I find it odd that our biggest capitalists at times fail to recognize what motivates the basic employees. Each successive younger generation has become less loyal to the boss man and that trend down has followed what wages have done in comparison to productivity. "Well Crazy, robots and technology made people more productive..." Totally fair, so now people get more done in the 8 hours we have forced them to work since Ford was having them build the Model T but pay has not kept up. You will have periods where you have a highly motivated employee that you can keep motivated by the reason they are there. Money. However, eventually you run into a wall where you can't meet their expectation. Those who can will go find someone who will pay what they want and those who cannot will settle in after some point in time and do what you pay them for. If you have a bunch of employees that generally means they drop to just above average.
I'd also guess that if you're a completely transactional type employee focused on only doing a very specific assigned task, you risk being replaced that much more easily by AI or tech or whatever that is cheaper. If you're the type who has a more general outlook like I described above, a good employer will do anything they can to keep you.
If we are being honest, that kind of attitude is kind of why employees check out. "Why are you approaching this transactionally?" asks the owner or CEO.
"Well boss, I am doing a good job right?"
"Yes, you are. One of the best I have."
"Great, it would mean a bunch for my work/life balance if I continue to be your best employee, my pay doesn't change, but I only stay in the office 7 hours a day instead of 8. I will do everything to make sure that I reduce time wasters in the day so my production won't fall...."
"No, no, I need you the 8. If you can get it done in 7 then there is even more I can get out of you. Why would I pay the same for less time? And if you don't former great employee, well look out for A.I."
Yeah, you're transactional too boss. We all are. We get paid by you to do a job. Nobody is working for free. You don't pay McDonald's prices and epect Michelin service. Laborers are capitalists too. When you hire a plumber to do a service, he charges for every thing he finds above and beyond the initial call. Owners expecting 100% from everyone without having a compensation structure that accommodates for that end up where I said they do. It is a very small percentage of people that will continue to be their absolute best when they lose the carrot to keep being that way.