(so, I edited this to include the other theories I started to develop in comments. PMar's answer states the "obvious" but is actually quite plausible. So, theories to follow are assuming Hogwarts could have spares, but just won't.)
(after-writing note: so it's quite long, and sometimes biased, but it was fun to think and write about...)
Because they wouldn't work as well as one's original wand.
I think they wouldn't work quite well, and a working wand seems to be essential for studying magic.
"The wand chooses the wizard" (Ollivander in Harry Potter and the Philosopher's Stone)
In this answer, you can see how a wand that didn't choose you just refuses to work properly. Unless Hogwarts was to buy a workload of spares to gamble with statistics (which would be expensive), it's unlikely each student would find a spare wand that fits him. At this point, you could even have Ollivander set a branch in Hogwarts.
They might deprive new wizards of wands.
So there's this eleven-years-old kid who goes to Ollivander's after receiving his letter. He's been treated like a freak for all his life by his Muggle family and can finally see the light upon knowing that he's part of an other universe, one of magic, full of promises - you get the idea.
So the kid's been trying dozens of wands at Ollivander's, but there's just nothing, you know? And the last one in the shop still doesn't fit him. "Isn't there another?," the kid asks Ollivander. Ollivander replies that no, it was the very last one, he's sorry, but a teacher came before him and bought the yearly amount of school spares,picking at random. Maybe his was in the lot? The kid gets a little hope and expects to find his at Hogwarts, but then he's told that he can't go to the school without a wand. So he goes back to his Muggle unloving family. How do you sleep at night, Hogwarts staff?!
Seriously though, if there's really only one wand that will accept to be your first wand (statistically unlikely, but hey, why not), you do not want anyone to buy a spare one "just in case". Also, it's even a statistical wonder that all these wizards find the perfect match. And that Ollivander, which we can assume makes wands partly randomly, still manages to create "the one" for nearly all the English wizard community. But I digress...
A wand is seven galleons, as reminded by Simpleton. There are about 280 students in Hogwarts, according to this source. Let's say that we account for 10% of spares. That's more or less 200 gold galleons. What's to do with 200 gold galleons? Hogwarts seems pretty filthy rich as is, maybe they'd have 200GG to spare on spares (pun not intended).
I don't really have an answer for this. I could say something along the lines of "well, just because a school has funds, doesn't mean they'll spend it wisely (source: all the years I've spent as a student)", but it's a lame answer. So because I don't like to be speechless, and because OP said he loved my headcanons, and because there's a ton of others things I should be doing, but won't, and because it'll serve to introduce a neat transition, I'll say this.
Hogwarts uses the spare money to pay off newspapers and the Ministry itself, in order to continue to exist. Given the events that happen each year, you'd think a lot of parents would like their kids to be homeschooled (wasn't there something about parents retrieving their kids from Hogwarts in Deathly Hallows?). And, you'd think that some anti-Hogwarts people at the Ministry would like to fire the whole staff, even close the whole school, and start a new one somewhere else, which they'd have full control of.
Hogwarts just doesn't care if you stay alive (except if your name is Harry Potter).
I know, I know. Hogwarts is "the safest place you could think of", or something like that. True, the place itself isn't bad, apart from the moving staircases, rather low gates on the astronomy tower, etc.
The staff is nice and kind. It's true that many professors will try to save your life when you're faced with deadly beasts they've put in the castle themselves. Oh, and from other teachers as well. Thinking about it, 200 gold galleons spared per year could be used to hire a HR person to recruit legit DADA teachers (legit, as in they're not Death Eaters, dangerous beasts, frauds, or Ministry harpies).
Well the place AND staff have flaws, but what doesn't? You'll make friends anyways, right? Oh, I forgot, some of the kids are quite messed up themselves, going around controlling giant snakes to kill little girls and then blaming their peers. And some of them come from families that worship evil himself, and are tasked to kill the Headmaster. Other than that you're good.
So if you as students' representative were to suggest to buy spare wands to the face of a staff that has no problems with letting you face beasts, questionable teachers, living in a hazardous place + hazardous surroundings (Forbidden Forest, merpeople much?), and if on top of that you're not Headmaster's favorite student, well, I think the best answer you'll get is something along the lines of "Meh".