As some people have noticed in the comments, not only would your intended answer be unacceptable, but so would the standard bull-- answer. When you apply for a job, you want to be able to honestly say that you think you will really and truly love the work you're doing.
For example, let's say three people apply for a job engineering new frobnicators for Gadzooks Gadgets Ltd. At the interview, all three are asked, "Why do you want to work for us?" The first person comes in and says,
You know, I just want this job because I want some more money. It looks like the pay is alright, and making things isn't that hard, I guess.
The second person comes in and says,
I want a chance to improve my professional career and learn about new opportunities.
And the third person says,
I went to school and studied frobnication engineering for four years. I started out in electrical engeering, but when I learned that frobnicators use gyroscopes, I realized I was truly interested in them. Since I got my degree, I have taught volunteer summer camps teaching kids about precession and have attended every frobnication trade convention I could afford to go to. At these conventions, Gadzooks Gadgets kept on coming up as a great place to work, so when I found out you had a position open, I knew I had to apply right away.
Who cares whether person one wants to make money or person two, deep down, truly wants to grow professionally? Person three's desires for his or her job line up perfectly with what the job involves, meaning that he or she is both going to be excellent at it and he or she is going to love the work, which is good for everyone involved.
So let's say you're not person three. You're applying for a lot of jobs that you can't honestly say you get fired up over. Why not? More specifically, why aren't you looking for work in the field that interests you?
Suppose our third person started learning about electrical engineering in school, decided to switch to fronbication, and then later switched back to electrical engineering, deciding that circuit diagrams are much more enticing than gyroscopes. At this point, the work that's going to keep this person the most motivated and effective is working with electrical components, not frobnicators. If he or she then applies for the position at Gadzooks Gadgets, what is he or she going to say about frobnicators? Probably something like,
I want this position because I think it will be a chance to improve my professional career and learn about new opportunities.
And then if person three gets the job, he or she will be stuck working on frobnicators all day, wishing he or she could go home and play with curcuit diagrams instead. If he or she had applied for the position designing new products for the company manufacturing power plant equipment instead, he or she would have been much happier and would have had an easier time with the interview.
So the long-term answer for your question is, what is it that you would really enjoy doing as work? What kind of job would you be able answer the question, "Why do you want to work for us?" with, "Because the stuff you're doing truly excites me"? And is there any particular reason you can't focus your job search in that field?