Simply because that's your job to create a solution that answers user needs, and it's expected you know better than they do, otherwise they would get your job.
I guess I should begin by saying that listening to their requests can be a good thing. Because users aren't complete idiots, they're well aware of their problems and needs seeing how they've been facing them every day for years, and are perfectly able to find a solution to it, no matter how bad it can be. So when you ask them what they need, they'll tell you they want a solution, and begin presenting what they think is the solution and tell you to make it. And honestly, I've sometimes done just that, precisely because they aren't complete idiots. Their propositions deserve to be analyzed in depth, for they know and understand their job better than you do. Their proposed solutions can be very relevant, and I sometimes couldn't find better myself after looking at their real needs. Most importantly, because of the different thought process they go through, their solutions can reveal additional things about their needs that they wouldn't express otherwise.
But you know things they don't. Hopefully. Their knowledge is shaped by what they've experienced, by the structures, the architectures, the software they've encountered, and when they think of a solution, they assemble those pieces they've seen work into something they think would work. But they're not designers. You know more of those pieces than they do, and can assemble them much more efficiently, doing things they couldn't even think of. People with many varied experiences can be as good at this as you are and are definitely worth listening to, but others will tend to make bad, inefficient, limited solutions.
Then, users rarely look at the big picture. They mostly focus on their desk jobs, and while fully aware they're just a cog in the machine, they usually see no further than the neighboring cogs. Because their vision is limited, so are their solutions, because all they want is you to improve their job. Because they like their job, and they don't like change. And they want to keep that job. They would hardly suggest you something that would make their own job obsolete, even though that would greatly improve the company's operations, because they're afraid they might lose their job in the process. Some of them may see you as a potential enemy, and will try to manipulate you into doing things that would suit them, and you'd better not listen to these shifty people. Thus their requests may very well be detrimental to the overall process, and might be counterproductive.
Finally, users mostly think about the functional aspect of things. Actually, I've never seen a user think about anything else than that. They don't realize the technical considerations, nor time and money constraints behind their requests, because that's an aspect they're never confronted with.
While those are to me the main reasons why you're better at your job than they are, we could certainly find more, but I guess you see the point. While listening to requests can certainly be useful, you're simply a better designer than they are. After all, product design is within the reach of anyone, it's just that some are better than others.