Spells are a non-renewable resource. One a spell has been cast, it can never be cast again.
But thankfully, what counts as a unique spell is permissive, and very early on in the history of wizardry, wizards found many ways to use the arcane language to specify a similar effect even if the wording was different.
And still, spells were a non-renewable resource.
There are only so many ways to call forth a beam of lancing light, only a limited number of methods of purifying food to make it safe to eat. Soon it became necessary for the wizards to start casting spells that weren't quite what they wanted: a beam of light that arced to the left, a purifying spell that added a bitter taste, some changes cosmetic and others very functional.
And still, spells were a non-renewable resource.
Wizardry was divided into ages by the historiographers. The First Age was the age of plenty, when wizards could make minor tweaks to the spells and cast as much as they liked. The Second Age was the age of modification, when wizards were jumping through hoops and using methods with side effects. But the Third Age was the age of decay, when so many spells had been used that only the oddballs were left. It was impossible to cast anything even remotely resembling a fireball, not even one that hooked to the left and exploded with sharp green shards.
It came to be that few wizards could produce a spell on their first attempt. They would try, only to discover that someone else had already taken their idea and the spell does not work. They would try again, only to discover that their second idea had also been taken. Wizard battles, which had once been glorious light shows, were reduced to two wizards standing in a field trying to be the first one to stumble upon a spell that had never been cast before.
Here are some plot hooks:
Wizards jealously guard their knowledge, fearful that someone will learn of a "seam" of untapped spells, but they also write down every spell they know to have been cast, to reduce their search space. Obviously this trove of knowledge is highly valuable.
The existence of spell "seams", which are really just collections of spells that work off the same cluster of discrete variations, mean that wizards tend to be very specialized. The Sheep Wizard knows eight hundred ways of turning someone into a sheep, because he's studied that area of the arcane language extensively, as well as historical precedents that have been ruled out. The natural enemy of a Sheep Wizard is, of course, another Sheep Wizard.
During the Second Age, a group of wizards get together to deliberately reduce the spell-space, largely in the hopes of reducing the capacity of wizard-kind for making war. Their work largely consists of sitting around casting as many fireballs as they can, depleting all options for everyone else.
During the Third Age, a group of wizards gets together and in the spirit of mutual cooperation begins to define "spell blocks", a collection of spells that a single wizard is entitled to and all other wizards agree not to use. When you become a wizard, you're given a thousand spells which are thought to still be valid, and will lose your license to practice wizardry if you cast any spells that are outside your block. This is difficult to enforce, rife with accusations and suspicion, but is thought to be better than nothing.
During the Fourth Age, a group of "wizards" (none of whom have ever actually cast a spell) are working on the arcane language in the hopes of a revival. As the age of hoarded knowledge has mostly passed, they're able to get their hands on many books that weren't previously available. One day, they invent a new form of specification that allows hundreds of thousands of new spells, re-igniting wizardry.
