This is a consideration of the WIZ attribute and I shall centre it upon humans.
Why is Trollworld not bursting at the seams with human wizards? At first glance it ought to be! How so?
The average roll of 3d6 is 10.5 and so most humans should pass the 1st Level wizards' test - 10 for INT and 10 for DEX. Ah but maybe no one teaches them and that's why so may are just citizens. I don't believe this is so...
The Wizards' Guild was formed to counter the law-breaking of people with the ability to cast spells. The idea was that spells would be standardissed and social engineering would go hand in hand with magical tuition in classes so that graduating wizards would keep the law, by and large, and the rulers would be better served.
It wasn't a matter of money - the rulers wanted to net as many fish as they could. Rogues are literally rogue wizards - people who the trawling nets have missed and have not been 'programmed' magically or ethically. (I see no reason why they should have just one spell or that the spells they have should be just as those the Guild teaches but that's another story.)
The explanation for the comparative lack of wizards, given that the INT and DEX starting requirements are so modest, might be this: there are two types of WIZ.
The common one gives rise to spell resistance and everyone has it to some degree, even though it may be very weak. The rarer version allows spell casting. The Wizards' Guild attempts to assess every citizen and then trains those who have the rarer WIZ quality.
That's how it works on Khaghtch'an, the Kraken Continent, anyhow.