There are so many magical-combat mangas these days, most with high school thrown in. The first one I encountered was Freezing, but I see so many of them, like http://www.mangaupdates.com/series.html?id=109987 .

I don't like most of these. Purely personal opinion. Are there so many because the genre is wildly popular, because you don't need to come up with a story line, just flashy battles, or some other virtue that hasn't occurred to me? Help me understand.

It's probably just as you've said. I mean all three of those categories (magic, combat, and high school) are ridiculously popular anyway so it not really that surprising is it?

That's why if you see a description of a manga that seems completely inane or uncommon premise with awful artwork, you'd better read it because it must have earned its virtue in the plot.

There must be a large demand for them. They're not to my taste either though.

It's like asking, why are there so many high-school romance shoujo mangas. For starters, highschool magic-combat genres are very broad mainstream genres to begin with. For starters, magic basically needs combat, otherwise you're reading a manga of david blaine...while the high-school genre is to satisfy a massive demographic, the shounen demographic. Shounen mangas to begin with typically involve teenage characters (to make it relatable) and is pretty much either slice of life, rom-com, or magic-combat.

So why is there so many magic-combat high school mangas? Because magic-combat and high school, are two enormously popular subjects to begin with.

For starters, why do people read manga? It is for the sake of entertainment, and while realistic slice of life drama series have a certain allure to some demographics, to imaginative teenage boys who love video games, magic and combat fiction is much more interesting. It's why the big name shounen's like Dragon ball Z, Naruto, Bleach, One Piece are all supernatural action genres. It feeds their imaginative curiosity, satisfies their desire for excitement, and lures them in with new powers and abilities.

It is the exact same reason why Harry Potter is so popular. It takes the "essentially normal" child, and grants them magic powers, immersing them in a fictional world that brings out excitement and action. How many boys and girls wished that magic was real, that they were some special wizard?

It's understandable that you don't like most of the high school magic-combat series, after all, as such a popular series it is bound to have lots and lots of mediocre mangas. You'll find many of these series do not have many volumes, and many reach hiatus/cancel status quite rapidly. The reason why is authors are trying to create the next DBZ, the next Naruto, etc. They tweak some subtle plot settings, shinigami, versus magic, versus swords, versus science magic, versus etc, and rework similar characters, the buffoon, the prodigy, the tsundere girl, the childhood friend. They throw it all together into a storyline, and depending on how the audience reacts, the series continues or is cancelled.

When one fails, they start up a new one. When that fails, they start up a new one. And finally out of the hundreds that are made, one finally climbs up to the top and is immortalized as a 200+ chapter series.

It's really not about originality, coming out with complex or unique plots, or coming up with super realistic characters, it's about combining several elements together to make a series that the general populace will enjoy. They're not meant to be masterpiece series, they're meant to be mass-produced general consumer goods.