In an ideal world, you would because you would care for people. If not everybody, at least the people around you. As an employer, you are hiring people who will spend the majority of their waking hours working for you, with you, at your company. In theory, you would care if they are happy or not, motivated or not, paid fairly or not. You would care if they can afford that visit to the doctor they need, if they are taking enough breaks, enough time off. Ideally, you would keep an eye on your employees to make sure nobody is overdoing, to watch out for for employees whose performance is going down and figure out if they are getting burnt out or what's actually going on. All those things would pay back in the end. Happy employees do better works. Burnt out employees don't. Recovering from that is hard, and even though you can say "who cares, I'll fire them and hire someone else", there's still the fact that the hiring process is expensive.
In an idea world though. But I guess I do understand that for many people the only thing that matters is squeezing as much money for their business out of society as they can get away with. Though, this becomes a somewhat philosophical discussion at this point, and I think that's beside the point.
In my case honestness would have made several firms more profitable. I'm a high school drop out but lifelong programmer from a early age. As such I've had to work for well below market rates on my way up. Generally I do better than most of my college educated peers but since I lack the education I have been compensated much less. As a result I leave jobs every year or two, often around the time I'm really hitting my stride and providing significant value to the organization. As I tend to be above the median I end up on the projects that are critical to the company so what you end up with is someone building a quarter of your infrastructure and then leaving because "its corporate policy to max raises out at 5%" or some other nonsense. At that point you have to go back to the market (counter offers when you told me raises are warranted but impossible are an insult and I refuse to accept them) and likely pay at least market rate for a new employee then get him or her up to speed all as deadlines slip.
When I explain this to whomever I'm giving my notice to I am frequently asked "why would I pay you more than I need to" as a retort. I just shrug my shoulders and move on to the next job.
It kind of illuminates the whole system.
The fact that business shouldn't need to pay a fair, or even a living wage (in fact, they're kind of obligated not to, if possible, because, competition).
It works as long as there are people desperate enough to take any offer at all - which will always be the case because people need to eat, so they can't opt out because wages are too low. So not really a "free market" at all.