I struck up a conversation with another young teacher today. He's been teaching math for five years, and he's just starting at this school district. We were talking about the difficulties of first year teaching: how every class is new and you're doing everything for the first time. I told him that I'd hoped that I would be able to reuse some of the materials I planned for my classes last year. I won't actually be able to, since I'm teaching completely new classes now.
He said that in his five years of teaching, he's had new classes almost every year, so he knew how I felt. He was never able to reuse any of his old lesson plans. Yearly staff changes at his former school district ensured that he'd get a totally new schedule each year. We were both in the same situation. We had prepared other classes, but we wouldn't be able to use those lessons for our classes this year.
Then he surprised me. He said, "I can pretty much just wing it now and teach a lesson on anything. What are we learning about today? The Pythagorean Theorem? OK! Let's go!"
Wow. I'd never be able to do that in business classes. No way. I can see how it would be possible to do this with mathematical concepts, which are basically procedural knowledge rather than factual knowledge. But to just "wing it" for a lesson on globalization, labor unions, or contract law? That's not even possible. Yeah, I could explain the concepts well enough, just winging it, but that wouldn't be much of a lesson.
A good lesson in topics like these must include other resources: newspaper clippings, online articles, worksheets, or videos, for example. Finding and preparing these resources is very time-consuming. This is a task that math teachers can skip (although I'm sure there are some superb math teachers who do this type of research as well).
In math, students don't learn these skills by passively listening. You can't learn to multiply just by watching someone else; you need to practice on your own. Math textbooks provide drill after drill for students to practice these skills. Teachers of math simply need to set aside some work time, assign some textbook drills, and provide individual help as needed. Very little preparation is required.
Business students learn by engagement too. They don't learn how to perform a SWOT analysis just by reading about it; they need practice too, just like the math students do. Only, business teachers can't just have students do repetitive textbook drills like the math teachers do. Teachers must seek out or create their own engaging assignments. I like using debates, essays, research papers, role plays, and even skits as reinforcing activities. Business teachers have to spend much more time preparing these types of activities than math teachers spend preparing drills.

There are a few caveats, of course. In my programming classes, I can "wing it" and teach loops and switch statements just as easily as math teachers can "wing it" and teach the Pythagorean Theorem. If I have a good textbook, I can assign exercises instead of writing my own. Teaching a class like this is pretty nice; the prep work is much less.
Also, I know that there are stellar math teachers who do all of the same research and prepare all of the same activities that I do for my business classes. Math is about much more than just learning rote procedures. I've observed some math teachers in class, and I can tell that the good ones put a lot of work into their lessons. A teacher's choice of textbook makes a big difference, too. If you're lucky, you'll find a textbook that aligns nicely with your course objectives and includes supplementary materials for teachers.
But, I do believe that
some subjects simply require more time to prep than others. Some teachers get this condescending tone of voice when they tell you that "teaching is easy" or "I can just wing it". They say this as though they are such expert teachers. If you're just winging it, you're letting your students down.
If you think teaching is easy, you're not doing your job (I don't think teaching is hard; I'm just saying that preparing good lessons is a lot of work). Even if you've been teaching your whole life, there are always things you can improve.