Human's aren't rational and they're inherently social. As much as you are right that you can really think of it as a monetary transaction most jobs and workplaces go beyond that. You have social relationships with people you work with. You work on tasks not merely because you get paid for it but because you genuinely want to make things better.

And it's not just employees that are loyal; I've know of many companies that go above and beyond for their employees when they need it. I definitely know companies that keep around long-term employees out of a sense of loyalty when younger employees would be both cheaper and more productive.

It's not completely black and white.